In a large twin series the monozygotic pair concordance rate for schizophrenia was found to be 3.3 times greater than the dizygotic rate. In contrast, the difference for psychoneurosis was only 1.3. Reanalysis of 18 earlier major twin studies revealed a similar difference in all but one instance. This consistent difference suggests the presence of a genetic factor in the pathogenesis of schizophrenia and its relative absence in psychoneurosis. However, the role of the suggested genetic factor appears to be a limited one; 85 percent of the affected monozygotic pairs in the sample were discordant for schizophrenia.