The authors performed a systematic diagnostic review of the records of all twin pairs with a psychotic diagnosis in the Veteran Twin Registry. They found 274 pairs of twins (313 individuals) in which one or both were "clearly" schizophrenic, a frequency of 0.98 percent. The monozygotic (MZ) concordance was 27.4 percent, the dizygotic (DZ) concordance was 4.8 percent, and the MZ/DZ ratio was 5.7. Compared with the results of a computer search, these results present a higher MZ concordance and MZ/DZ ratio. The authors also review the results of the major twin studies of schizophrenia, which indicate that both genetic and environmental factors are significant in the occurrence of schizophrenia.