Cerebrospinal fluid aminoacid analysis in a girl with severe psychomotor retardation, hypotonia, hyperreflexia and growth acceleration showed highly increased levels of free gamma-aminobutyric acid (4.8 mumol/l; range in twenty controls 0.04-0.12, median 0.08), homocarnosine, a dipeptide of gamma-aminobutyric acid and histidine (23.4 mumol/l; control range 4.0-8.7, median 7.6) and of beta-alanine, an alternative substrate for gamma-aminobutyric acid-transaminase (0.48 mumol/l; control range 0.02-0.06, median 0.05). Liver gamma-aminobutyric acid-transaminase activity was deficient (0.07 mumol/mg protein h; range in ten controls 0.31-0.69, median 0.38). Fasting plasma growth hormone levels were increased (7.9-38.4 ng/ml; nl less than 5). Brain evoked responses were suggestive of leukodystrophy. A brother of this patient, showing a similar clinical picture, had died at one year. Postmortem examination of his brain showed leukodystrophy of the type seen in amino acidopathies such as phenylketonuria. This appears to be the first report of gamma-aminobutyric acid-transaminase deficiency.