Obesity has been related to increased risk of colon cancer or adenomas, but the epidemiologic findings are not entirely consistent. We examined the relation of not only body mass index (BMI) but also waist-to-hip ratio (WHR) and weight gain to colon adenoma risk in men who received a preretirement health examination at the Japan Self Defense Forces (SDF) Fukuoka and Kumamoto Hospitals during the period from 1995 to 1996. In the series of 803 men at age 47-55 years, 189 cases of colon adenomas and 226 controls with normal total colonoscopy were identified. Weight at 10 years before was ascertained by referring to the recorded data. After allowance for hospital, rank in the SDF, smoking and alcohol use, weight gain over the past 10 years was significantly associated with increased risk of colon adenomas (odds ratio for > or = 6 kg versus < or =-2 kg = 2.2; 95% confidence interval 1.0-4.8). High BMI and high WHR were each associated with increased risk, but only WHR was related to the risk independently of weight gain. In particular, weight gain accompanied with a high WHR was associated with a significant increase in the risk. Men with high physical activity tended to have lower risk. Associations with obesity-related variables and physical activity were not materially differential as regards the location and size of adenoma. The findings indicate that weight gain in middle age leading to abdominal obesity increases the risk of colon adenomas, and consequently of colon cancer.