Retrospective studies of the intellectual performances of children who later became psychotic adults have yielded evidence of early interference in the development of intelligence in future schizophrenics. The intellectual assessments of 153 children were examined during two test periods in the St. Louis Risk Research Project. In 1967-1972, the Wechsler tests of intelligence were administered to offspring in families with one schizophrenic parent, one parent with affective disorder, one parent with schizoaffective disorder, one physically ill parent, or to offspring with two normal parents. In 1975-1978, the intelligence of offspring was tested again at a mean age of 16 years. Differences between children as a function of parental diagnoses were assessed and yielded significant time effects. Children of schizophrenics and children of parents with affective or physical disorders had greater loss in verbal IQ scores between the first and second testing than did the children of schizoaffectives and children of normal parents. Offspring of psychotic mothers had lower IQ scores than those of psychotic fathers. Children of schizophrenics and children of schizoaffectives had the lowest stability in IQ scores from the first to second testing. The findings suggest a need for further replication research on the effects of ill parents on their children. (Author/JAC)