Stafford Leak Warren (July 19, 1896 - July 26, 1981) was an American physician and radiologist who was a pioneer in the field of nuclear medicine and best known for his invention of the mammogram. Warren developed the technique of producing stereoscopic images of the breast with X-rays while working in the Department of Radiology at the University of Rochester School of Medicine.

Warren was commissioned as a colonel in the United States Army Medical Corps in 1943 and appointed Chief of the Medical Section of the Manhattan Engineering District. He was responsible for the health and safety of the thousands of personnel involved in the Manhattan Project. He was present at the Trinity nuclear test in Alamogordo, New Mexico, where he was responsible for the safety aspects of the detonation of the world's first nuclear weapon. He led a survey team from the Manhattan Project to assess the effects of the atomic bombings of Hiroshima and Nagasaki. In 1946 he was Chief of the Radiological Safety Section of the Joint Task Force for Operation Crossroads, the nuclear test at Bikini Atoll.

In 1947 Warren became the first dean of the School of Medicine at the University of California, Los Angeles (UCLA). Under his leadership, the new school grew from nothing to a major medical school. Although controversial and opposed by James Bryant Conant, Warren would speak out about the dangers of nuclear fallout from weapons testing from about 1947 onwards and upon the Castle Bravo test in 1954 his views were lent some credence, which informed opinion leading up to the Partial Nuclear Test Ban Treaty. Warren became Vice Chancellor, Health Services in 1962 at UCLA. From 1963 to 1965 he served as special assistant to President John F. Kennedy and later Lyndon B. Johnson on mental retardation. Returning to UCLA in 1965, he became professor emeritus, a position he held until his death in 1981. In 1971, he was awarded the Enrico Fermi Award for his contributions to science and medicine.

Stafford Leak Warren was born in Maxwell, New Mexico, on June 19, 1896. He attended the University of California, Berkeley, where he earned his Bachelor of Arts degree in 1918. He married Viola Lockhart on May 22, 1920. Their marriage produced two sons and a daughter. He went to medical school at the University of California, San Francisco, graduating with his Doctor of Medicine degree in 1922. He then did post-doctoral work at Johns Hopkins School of Medicine and Harvard University.

Warren joined the faculty of the Department of Radiology at the University of Rochester School of Medicine in 1926, becoming an Assistant Professor of Medicine. The department was a new one that had only been founded in 1921, based upon the ideals of medical education propounded by Abraham Flexner in his 1910 Flexner Report. The medical school had opened in 1925 and did not graduate its first class until 1929. Warren was one of the original group that Dean George Whipple assembled to staff the new medical school. At Rochester, Warren investigated the effects of artificial fever, including its possible use for treating gonococci infections. He was also interested in radiography.

In 1930, Warren became an Associate Professor of Medicine. That year he published "A Roentgenologic Study of the Breast". X-ray imaging of the breast had first been investigated by Albert Salomon at the University of Berlin in 1913. He produced images of over 3,000 mastectomy specimens, and observed different forms and stages of cancer. He was even able to detect an undiagnosed cancer in a breast that had been removed because of a large cyst. However, Salomon apparently did not appreciate the life saving potential of his observations. Warren used radiology to track changes in breast tissue as a result of pregnancy and mastitis. He developed a stereoscopic technique, using patients rather than specimens and adapting general purpose X-ray equipment to his needs. His method involved having the patient lie on her side with one arm raised while being X-rayed. In 119 women who subsequently underwent surgery, he correctly found breast cancer in 54 out of 58 cases. This represented a leap forward, as it made diagnosis of breast cancer possible without surgery. It also sparked intense interest in the technique around the world. Today mammography saves thousands of lives.

In February 1943, Warren met with Dr Albert K. Chapman, the vice president and general manager of Eastman Kodak, Brigadier General Leslie R. Groves, the director of the Manhattan Project and Colonel James C. Marshall, the commander of the Manhattan Engineer District (MED). They asked him about his work with radiation and isotopes and asked Warren to become a civilian consultant for the MED. In March 1943, Warren met with Major Hymer L. Friedell at the MED's New York City headquarters. Friedell had joined the Metallurgical Laboratory in August 1942 from the University of California, Berkeley, where he had been involved in radiology research under Dr Robert S. Stone. Friedell requested information about safety matters involving radiation. Warren was unable to provide all the information, but agreed to initiate research at Rochester.

Friedell recommended that in view of the scope of the Manhattan Project, a more senior officer be placed in charge. In October, Groves penned a letter to the Surgeon General of the United States Army, Major General Norman T. Kirk, requesting that he cooperate with the Manhattan Project in furnishing medical supplies, providing funds for medical care of military personnel, and directly commissioning Warren and other doctors at specified ranks. This letter was signed by Lieutenant General Brehon B. Somervell and delivered in person by Colonel Kenneth D. Nichols, who had replaced Marshall as the district engineer. Kirk was furious at what he saw as usurpation of his statutory responsibilities as Surgeon General. When told that Warren was the man the district had in mind to commission as a colonel, and apparently familiar with Warren's work only from his use of radiation to treat venereal disease asked "Why do you want that clap doctor?"