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Data collected from Nisei men and women, offspring of immigrants to the United States from Japan, were examined for evidence of possible genetic heterogeneity in Japan with respect to type 2 diabetes or non-insulin-dependent diabetes mellitus (NIDDM), impaired glucose tolerance (IGT), and hypertension. The subjects were 391 men and women with a mean age of 62.0 (±0.3) years. Patterns of disease expression in the Nisei with respect to the origins in Japan of their parents indicated that the genetic basis for NIDDM may be more frequent in northern Honshu than in southwestern Honshu, whereas that for IGT may be more frequent in southwestern Honshu. Further analyses indicate that the pattern for IGT is restricted to men. Hypertension appears more frequently in persons with parents from northern Honshu and less frequently in women but not in men from southwestern Honshu. For men an analysis of age and family history of diabetes by oral glucose diagnostic category revealed the presence of a group of younger men with IGT but, surprisingly, no family history of diabetes. Thus the data show an apparent lack of the consistency expected if diabetes and IGT simply represent stages of one disease entity. We suggest that IGT may represent a heterogeneous category including both an early or transitional stage of NIDDM and another condition found primarily in men in which less severe glucose tolerance appears and with which hypertension may be associated. Data on ancient settlement in Japan suggest a possible historical basis for the patterns found.