In 1984, the Japan Diabetes Society organized a committee to collect data on diabetic twins in Japan. Within 3 years, through correspondence with Society members and hospitals, the Committee had contacted 87 pairs of twins, one or both of which had diabetes mellitus or glucose intolerance. Sixty-three pairs were monozygotic and 24 dizygotic. The probands, who had been diagnosed as diabetic or glucose-intolerant earlier, included 21 patients with insulin-dependent diabetes mellitus (IDDM), 56 with non-insulin-dependent diabetes (NIDDM), one with an unknown type of diabetes, and nine with glucose intolerance. Concordance between monozygotic twins was 45% for IDDM and 83% for NIDDM; between dizygotic twins, concordance was 0% (0/10) for IDDM and 40% (4/10) for NIDDM. Concordance was significantly greater in NIDDM than in IDDM, and in monozygotic than in dizygotic twins. Concordance was greater among twins in which one twin had developed diabetes after the age of 20 than among twins in which the age of onset had been earlier. There was no evidence that the period of discordance was shorter in the discordant pairs than in the concordant pairs. About 90% of the IDDM twin pairs lived together, against 20% of the NIDDM pairs, probably due to the later age of onset of NIDDM. The frequency of diabetes in family members other than the twins was higher in NIDDM than in IDDM regardless of concordance. In concordant pairs the presence or absence of various complications agreed in 68-97%; a few pairs were discordant for the severity of retinopathy, which may have resulted from differences in duration and hyperglycemic degree. A 75-g glucose tolerance test administered to the 'normal' discordant twins revealed borderline glucose intolerance in two of six IDDM co-twins and six of eight NIDDM co-twins. The early-phase insulin response was low in two of six IDDM co-twins and four of six NIDDM co-twins. These results are generally consistent with previous reports of diabetic twins in Western countries, reinforcing the importance of genetic factors in the pathogenesis of diabetes. Heredity has a particularly strong influence in NIDDM.