Al-Razi began compiling his most extensive medical work, "Kitab al-Hawi". This encyclopedic text contains a vast collection of medical knowledge, including his own observations, case studies, and critical analyses of other physicians' works. It was intended as a comprehensive resource for medical practitioners. It wasn't fully organized until after his death.

Al-Razi wrote his famous treatise on smallpox and measles, which provided the first clear and accurate descriptions of these two diseases and differentiated them from each other. This work demonstrated his keen observational skills and his ability to distinguish between similar ailments. It was translated into numerous languages and remained a standard medical text for centuries.

Besides "Kitab al-Hawi" and the treatise on smallpox and measles, al-Razi wrote numerous shorter medical treatises on a variety of topics, including anatomy, surgery, pharmacology, and specific diseases. These writings further solidified his reputation as a leading medical authority.