By the cooperation of members of the Japan Diabetes Society, we collected data on 144 twin pairs with diabetes or glucose intolerance. By the diagnosis of reporting physicians, the concordance rate of diabetes in monozygotic twins (MZ) was 47% (9/10) for IDDM and 87% (68/78) for NIDDM. The concordance rate in dizygotic twins (DZ) was 8% (1/13) and 43% (9/21) for IDDM and NIDDM, respectively. The concordance rate was higher in MZ than in DZ and in IDDM than in NIDDM. In MZ, the concordance rate was higher in twin pairs in which probands developed diabetes at an older age. The discordant period for diabetes was not shorter in discordant pairs than in concordant pairs. The period of living together was irrelevant to concordance for diabetes. When co-twins of NIDDM twins were examined by oral glucose tolerance test, 98% of co-twins of MZ and 92% of co-twins of DZ had diabetes or a milder degree of glucose intolerance, while in IDDM pairs 45% of MZ and 50% of DZ co-twins had normal glucose tolerance. The insulinogenic index (the ratio of the increment of insulin to that of blood glucose 30 min after a glucose load) in discordant NIDDM co-twins was similar to subjects with the same degree of glucose intolerance.