Twelve children and young adults with Shwachman syndrome were compared with their unaffected siblings and with controls suffering from cystic fibrosis in terms of intellectual ability, motor skills, and behaviour. There were highly significant differences in intelligence quotient between those with Shwachman syndrome and the other two groups. Four of the index subjects but none of the control subjects were below the normal range. The differences between groups on other tests of cognitive and motor skills were not significant, though those with Shwachman syndrome tended to have the lowest scores. There was no evidence that those with Shwachman syndrome had more behavioural difficulties than the control subjects. We suggest that the intellectual difficulties of patients with Shwachman syndrome may be of neurological rather than social origin and that they may originate before birth.