Homocarnosine, the brain-specific dipeptide of gamma-aminobutyric acid (GABA) and histidine, was found to be elevated in the CSF, i.e. approximately 20 times the mean control level, in two brothers and one sister. All three were similarly afflicted, i.e. with a progressive spastic paraplegia, progressive mental deterioration and retinal pigmentation. A sister was healthy, and there was no other occurrence of similar symptoms in the family. The clinical symptoms in the affected individuals seem to differ from those in other reported families. The unaffected sister, the father and two maternal aunts exhibited a normal CSF homocarnosine level, whereas the mother, who showed no definite clinical symptoms, showed a markedly elevated CSF homocarnosine level. The explanation for the latter finding remains obscure.