A cerebrospinal fluid leak (CSF leak or CSFL) is a medical condition where the cerebrospinal fluid (CSF) that surrounds the brain and spinal cord leaks out of one or more holes or tears in the dura mater. A CSF leak is classed as either spontaneous (primary), having no known cause (sCSF leak), or nonspontaneous (secondary) where it is attributed to an underlying condition. Causes of a primary CSF leak are those of trauma including from an accident or intentional injury, or arising from a medical intervention known as iatrogenic. A basilar skull fracture as a cause can give the sign of CSF leakage from the ear, nose or mouth. A lumbar puncture can give the symptom of a post-dural-puncture headache.

A cerebrospinal fluid leak can be either cranial or spinal, and these are two different disorders. A spinal CSF leak can be caused by one or more meningeal diverticula or CSF-venous fistulas not associated with an epidural leak. A spontaneous spinal cerebrospinal fluid leak may occur sometimes in those with predisposing heritable connective tissue disorders including Marfan syndrome and Ehlers–Danlos syndromes. A loss of CSF greater than its rate of production leads to a decreased volume inside the skull known as intracranial hypotension.

Any CSF leak is most often characterized by orthostatic headaches, which worsen when standing, and improve when lying down. Other symptoms can include neck pain or stiffness, nausea, vomiting, dizziness, fatigue, and a metallic taste in the mouth. A CT myelography scan can identify the site of a cerebrospinal fluid leakage. Once identified, the leak can often be repaired by an epidural blood patch, an injection of the patient's own blood at the site of the leak, a fibrin glue injection, or surgery.

A spontaneous CSF leak is a rare condition, affecting at least one in 20,000 people and many more who go undiagnosed every year. On average, the condition develops at age 42, and women are twice as likely to be affected. Some people with a sCSF leak have a chronic leak despite repeated patching attempts, leading to long-term disability due to pain and being unable to be upright, and surgery is often needed. The symptoms of a spontaneous CSF leak were first described by German neurologist Georg Schaltenbrand in 1938 and by American neurologist Henry Woltman of the Mayo Clinic in the 1950s.

Cerebrospinal fluid leaks are classified into two distinct disorders: cranial leaks, and spinal leaks. Where there has been no preceding surgery or medical procedure that may have caused a CSF leak it is called a spontaneous CSF leak (sCSF leak).

Cranial leaks occur in the head, and in some of these cases, CSF can leak from one side of the nose, or from the ear. Intracranial hypertension is often associated with a spontaneous cranial CSF leak, rarely reported with a spinal leak.

The vast majority of CSF leaks are spinal. Spinal leaks occur when one or more holes form in the dura along the spinal cord. There are three types of spontaneous spinal CSF leaks. A spinal leak typically causes spontaneous intracranial hypotension.

Spontaneous intracranial hypotension (SIH) refers to lower than normal CSF volume due to a leak of CSF at the level of the spine.