Genetic and biochemical findings in paranoid schizophrenia and other paranoid psychoses are reviewed. Although the data suggesting a lower genetic loading for schizophrenia in paranoid versus nonparanoid schizophrenia are unclear, paranoid schizophrenia does, to a limited extent, breed true within families. Monozygotic twins concordant for schizophrenia tend to be either both paranoid or both nonparanoid schizophrenics. In all studies, the risk of schizophrenia in the relatives of patients with paranoid psychosis is close to that found inthe normal population. Genetic studies provide no evidence for a link between affective illness and either paranoid schizophrenia or paranoid psychosis. Although reports of low platelet monoamine oxidase activity in paranoid schizophrenia have not been confirmed, recent results suggest that brain norepinephrine levels may be higher in paranoid than in nonparanoid shizophrenics. Genetic and biochemical findings suggest some differences between paranoid and nonparanoid schizophrenia, but definitive clarification of the relationship between these two syndromes must await future research. From a genetic perspective, paranoid psychosis appears to bear little relationship to schizophrenia.