In this prospective study 8,006 Japanese men, age 45-68 years, were examined between 1965 and 1968. Their height and weight were measured and they were asked for their weight at age 25. After a surveillance period of almost 15 years, 646 incident cases of the following cancers were identified: 104 stomach, 101 colon, 101 lung, 96 prostate, 63 rectum, and 181 cancers of other sites. Body mass index (BMI = weight/height2) at time of examination and weight gain since age 25 were positively associated with an increased risk for colon cancer in subjects age 55 or older at the time of examination. No other cancer had a significant positive association with either BMI at the time of examination or with weight gain since age 25. In contrast, a low BMI at examination and weight loss since age 25 were associated with an increased risk for stomach cancer, whereas only weight loss since age 25 was associated with an increased risk for lung cancer. These two cancers accounted for the overall significant association of weight loss with total cancer incidence.