Sixty-eight second-generation Japanese-American (Nisei) men 45-74 years old and 26 Tokyo Japanese men 40-72 years old with type 2 diabetes were given 75 g glucose by mouth, and their venous blood levels of glucose and insulin were measured at 0, 30, 60, 90, 120, and 180 min. Blood glucose levels were similar for the two groups but the Nisei had higher levels of insulin at all time points (P less than 0.006). The Nisei men had a higher mean body mass index (P less than 0.001) and the Tokyo men had had diabetes for a longer duration (P less than 0.001). Data were also compared following selection of a subgroup of Nisei men (n = 39) with a diabetes duration of 2-25 years, making this group more similar to the Tokyo men with respect to this variable. The Nisei still had significantly higher plasma insulin levels. Because of a significant (P less than 0.01) positive correlation between plasma insulin and body mass index in Nisei, but not in Tokyo men, data comparing the Nisei subgroup and Tokyo men were re-examined by analysis of covariance. With an adjustment for body mass index in the analysis, all differences in plasma insulin, except for the fasting insulin level, were no longer significant between the Nisei and Tokyo men, suggesting that body mass index was responsible for these differences.