Excessive excretion of N-acetylaspartic acid in urine is reported in a 6-yr-old child with extensive and progressive cerebral atrophy. The concentration in urine was 947-1,433 mumol/mmol creatinine (controls, n = 10, 5-21 mumol/mmol creatinine) and the daily excretion approximately 3-4 mmol. In cerebrospinal fluid from the patient the concentration was 611 mumol/l (controls, n = 10, not detectable, detection limit 2.3 mumol/l). The concentration of N-acetylaspartic acid in serum was 7 mumol/l. The low level in serum compared to the high urinary excretion of NAA suggests the possibility that NAA is synthesized in the kidneys in addition to the brain. This patient may cast new light on the functional role of N-acetylaspartic acid in humans.