Family history and aging are independent risk factors for the development of hypertension as well as for the development of diabetes. However, it is unclear how the family histories influence the rate of age-associated increase in these diseases. Moreover, despite the fact that hypertension and diabetes often occur concomitantly, it is not known whether family history of hypertension increases the risk of diabetes or vice versa. To gain an insight into these questions, we investigated the cross-sectional prevalence and family history of hypertension and diabetes in 1,123 male subjects (mean age, 42.1 +/- 12 years; range, 20-60 years) who participated in annual medical check-ups. The data were analyzed by 10-year age groups (20s, 30s, 40s and 50s). The prevalence of hypertension increased with age group either in the absence (12% in the 20s and 39% in the 50s) or in the presence (21% in the 20s and 59% in the 50s) of family history of hypertension, and thus the increasing rate of prevalence with age was not affected by family history. The prevalence of diabetes in the absence of family history of diabetes was low until the 40s (< 1.2%) but it jumped in the 50s (4.3%). On the other hand, in the presence of family history, the prevalence was 4% in the 20s and progressively increased to 20% in the 50s. The impact of family history on the risk of diabetes was strong and appeared to increase with age. Family history of hypertension did not increase the risk of diabetes, and family history of diabetes did not increase the risk of hypertension. These results suggest that family history of hypertension has an additive impact on the age-associated increase in the risk of hypertension, whereas family history of diabetes has an exponential impact on aging-associated increase in the risk of diabetes.