The distinction of psychosis into schizophrenia and bipolar disorder has been increasingly challenged with some evidence suggesting that the two conditions may have common etiologic and pathogenic mechanisms. We compared the premorbid and current intellectual function of bipolar patients from multiply affected families, and those of their first-degree relatives, with those of a similar series of schizophrenic subjects, as well as their relatives, and normal controls. Only schizophrenic subjects showed lower premorbid IQ, suggesting that they, but not the bipolar patients or either relative group, had suffered neurodevelopmental impairment. However, both groups of patients had comparably poor current general intellectual levels, implying that some common pathogenic process operates once illness has begun. The two groups of relatives showed distinct differences in intellectual function but we cannot exclude the possibility that these were a function of our sampling methods.