OBJECTIVE: The authors sought to determine whether the clinical manifestations of schizophrenia and other psychotic disorders are correlated in affected sibling pairs. METHOD: They examined, in 256 sibling pairs concordant for DSM-III-R schizophrenia and 457 sibling pairs concordant for all nonaffective psychoses ascertained in the Irish Study of High-Density Schizophrenia Families, similarity for 1) symptoms, course, and outcome; 2) symptom factors; and 3) syndromes, defined by latent class analysis. RESULTS: Global course and outcome, as well as all major symptoms except hallucinations, were modestly but significantly correlated in sibling pairs concordant for schizophrenia. Three symptom factors-negative symptoms, positive symptoms, and affective symptoms-were all significantly correlated in concordant sib pairs. Latent class analysis suggested five schizophrenic syndromes. Class membership was significantly correlated in concordant sibling pairs. Similar results were found for sibling pairs concordant for nonaffective psychoses. CONCLUSIONS: The clinical manifestations of the schizophrenic syndrome (both narrowly and broadly defined) are moderately influenced by familial factors. From a familial/genetic perspective, schizophrenia as currently defined may be etiologically heterogeneous.