The authors present a 16-year update on schizophrenia in the National Academy of Sciences-National Research Council (NAS-NRC) Twin Registry. As of October 1981, a recorded diagnosis of schizophrenia was equally common in monozygotic and dizygotic twins. However, probandwise concordance for schizophrenia was significantly greater in monozygotic (30.9%) than in dizygotic (6.5%) twins. Biases in zygosity determination, diagnosis, or ascertainment could not plausibly explain these results. Correction for selection effects in construction of the registry produced concordance rates for schizophrenia approaching those found in previous studies. According to registry data, genetic factors appear at least as important in the etiology of schizophrenia as in several common medical conditions, including diabetes and hypertension. Results from the NAS-NRC Twin Registry support the etiologic importance of genetic factors in schizophrenia.