The relationship between DSM-III-R schizophreniform disorder, delusional disorder (DD) and psychotic disorder not otherwise specified (PD-NOS) and schizophrenia and affective illness (AI) remains uncertain. We explore this question in the Roscommon Family Study by examining symptoms, outcome and patterns of psychopathology in relatives. Probands were selected from a population-based case registry in the west of Ireland with an ICD-9 diagnosis of schizophrenia or AI. Personal interviews were conducted with 88% of traceable, living probands, a mean of 16 years after onset, and 86% of traceable, living first-degree relatives. Best-estimate diagnoses were made at follow-up. Schizophreniform disorder, DD and PD-NOS constituted 6.4%, 2.8% and 7.5%, respectively, of all probands with a registry diagnosis of schizophrenia. Probands with schizophreniform disorder had prominent positive psychotic symptoms, negligible negative symptoms and a good outcome, comparable to that seen in AI probands. Their relatives had an excess risk of schizophrenia spectrum illness but not AI. Probands with DD had prominent delusions but no other psychotic symptoms, few negative symptoms, fair to good outcome and an increased risk in relatives for alcoholism. Probands with PD-NOS had both moderate positive and negative psychotic symptoms, a poor to fair outcome and a substantially elevated risk in relatives of schizophrenia and schizophrenia spectrum disorders but not AI.(ABSTRACT TRUNCATED AT 250 WORDS)