The increased prevalence of obesity in Japan may contribute to the high prevalence of hypertension in Japan. In the present study, we calculated the odds ratio for hypertension in obesity (body mass index (BMI) >or= 25.0 kg/m(2)) using data from independent nationwide surveys conducted in 1980 and 1990. We estimated the percentage of hypertensives whose condition was due to obesity among total hypertensives in the general Japanese population. In the 1980 survey, 18.8% of 4,623 male participants were obese and 50.4% were hypertensive, whereas 22.6% of 5,893 female participants were obese and 41.1% were hypertensive. For both sexes, obese participants had a higher odds ratio for hypertension than non-obese participants (BMI < 25.0 kg/m(2)), and there was a significant dose-response relationship between BMI and the odds ratio for hypertension. Among all hypertensives, the percentage whose hypertension was due to obesity in 1980 and 1990 was 11.4% (95% confidence interval (CI): 4.7-17.7%) and 15.3% (95% CI: 6.8-23.1%) for men and 19.3% (95% CI: 12.1-25.9%) and 22.3% (95% CI: 14.6-29.3%) for women, respectively. Approximately 80-90% of individuals with obesity-induced hypertension were in the 25.0 <or= BMI < 30.0 kg/m(2) category for both sexes in each year. In conclusion, we found that obesity-induced hypertension as a proportion of total hypertension increased between 1980 and 1990 for both sexes. Obesity now is playing a more important role in the high prevalence of hypertension in Japan than it was before.