The conventional distinction between schizophrenia and manic depression has received little objective support from recent studies of phenomenology, outcome, or familial homotypy. Instead, much clinical, epidemiological, and morphological evidence suggests that within the broad range of Schneiderian schizophrenia there exists one form (congenital schizophrenia) that can be distinguished from other types, the manifestations of which are confined to adult life. We hypothesize that congenital schizophrenia is a consequence of aberrant brain development during fetal and neonatal life. Such patients show structural brain changes and cognitive impairment, and in their male predominance, early onset, and poor outcome, they reflect Kraepelin's original description of dementia praecox. We contend that adult-onset schizophrenia is itself heterogeneous. One important component is a relapsing and remitting disorder that is more frequent in females than in males, exhibits positive but not negative symptoms, and has much in common etiologically with affective psychosis. There also exists a very-late-onset group in which degenerative brain disorder is implicated.