Neuropsychological deficits are found in both schizophrenic patients and their relatives, and some studies have shown similar, but less severe, deficits in affective psychotic patients and their relatives. We set out to establish: (a) whether schizophrenia spectrum personality traits are more common in the relatives of schizophrenic patients than, in the relatives of affective psychotic patients; and (b) what the relation is between spectrum personality traits and neuropsychological deficits in these relatives. Relatives were interviewed using the International Personality Disorder Examination (IPDE), and also completed the National Adult Reading Test (NART), the Trail Making Test (TMT; Parts A and B) and Thurstone's Verbal Fluency Test (TVFT). Spectrum personality traits were equally common in 129 relatives of schizophrenic patients and 106 relatives of affective psychotic patients, but the performance of the former group was inferior to that of the latter on the NART and the TVFT. Relatives with high paranoid traits had lower NART scores than relatives without such personality traits; similarly, those with high schizoid traits took longer to complete the TMT, part B, than those without such traits; and relatives with high schizotypal traits generated significantly fewer words on the TVFT than those without such traits. We conclude that relatives of schizophrenic and affective psychotic patients share a propensity to schizophrenia spectrum traits, but relatives of the former have poorer neuropsychological performance. Furthermore, there exists an association between neuropsychological deficits and spectrum traits in both groups of relatives; in particular those with high paranoid traits have lower IQ scores than their less paranoid counterparts.