Our purpose was to perform the first systematic neurological, neurophysiological and psychological study of 18 patients with Cohen syndrome (MIM no 216550), aged 11 months to 57 years (median 27 years). All the patients had the essential features of this syndrome, i.e., typical facial and structural findings, mental retardation, microcephaly, ophthalmologic symptoms, granulocytopenia and a cheerful psychic disposition. Children with the syndrome were considered normal at birth, but upwards of 6 to 12 months, psychomotor retardation became obvious. The first symptoms were microcephaly, which manifested itself by the age of 6 months to 1 year, as well as hypotonia and delayed developmental milestones. Cohen children learned to walk at 2 to 5 years of age. Language development varied markedly. Neurological symptoms did not progress. All patients had normal EMGs. The three youngest (aged 11 months, 3 and 5 years) had normal EEGs, whereas the remainder had low-voltage EEGs. No irritative spikes or epileptoformic foci were found. Nine patients had quick beta transients. Of the 18 patients examined, 4 were profoundly, 11 severely, 1 moderately and 2 mildly retarded. On the AADM scale, Cohen patients had high scores in the positive domains, viz., self-direction, responsibility and socialisation. Maladaptive behaviour, on the other hand, was almost completely absent, except for stereotyped behaviours and odd mannerisms. Withdrawal, sexually aberrant behaviour, untrustworthy and rebellious behaviour as well as antisocial behaviour were rare. These findings are consistent with the cheerful and sociable disposition characteristic of those with Cohen syndrome.