In a prospective study, mortality ratios were computed in relation to overweight, cancer, and other diseases. The study included 750 000 men and women followed for 12 years. Each person was given a weight index. Death rates for overweight and underweight persons were compared with rates for persons of average weight. Men who were 40% or more overweight had a mortality ratio for cancer of 1.33; women, 1.55. This ratio was much lower than that for coronary heart disease (men, 1.95; and women, 2.07); diabetes (5.19 and 7.90), and digestive diseases (3.99 and 2.29). Overweight men had significantly higher mortality ratios for colorectal and prostate cancer; overweight women had much higher rates for cancer of the endometrium, gall bladder, and cervix; and also significantly higher rates for ovary and breast cancer.