It is unknown whether intellectual disability (ID) is more familially related to psychotic mood disorders or schizophrenia. L. S. Penrose's large sample of families with two or more members admitted to psychiatric hospitals provided a unique opportunity to investigate the familial relationship between mild ID, schizophrenia and psychotic affective disorders. Method There were 183 affected relative pairs comprising probands with mild ID (95 male, 88 female) and their first or second degree relatives with schizophrenia or psychotic affective disorder. Results There were nearly twice as many relatives with a diagnosis of schizophrenia (n=121) as relatives with affective disorders (n=62) among the intellectually impaired probands. This excess of schizophrenia was statistically significant, even after accounting for the increased risk of hospitalization for schizophrenia (P0.005), and was fairly constant across the different relative types. First-degree relatives with either mental illness were more likely to be parents (n=77) than siblings (n=51) or children (n=3), but there was no excess of mother-son pairs. Conclusions These results suggest a stronger familial relationship of ID with schizophrenia than psychotic affective disorder, and lend some support to the neurodevelopmental hypothesis of schizophrenia.