Mortality ratios by relative weight categories compared to average weights were computed for 750,000 men and women. Data came from the American Cancer Society's prospective study, conducted in 1960-1972. Males who were 40% or more overweight had a mortality ratio of 1.87 for all causes of death. The ratio in women was 1.89. High mortality ratios were also observed in overweight men and women who died of coronary heart disease, diabetes and digestive diseases. The mortality ratio for cancer in obese men was 1.33; 1.55 for obese women. Sites with high mortality ratios in obese males were colon-rectum and prostate; in women, endometrium (about 5 times as high as for average weights); cervix (2 times as high), gall bladder (3.5 times), ovary (mortality ratio of 1.6) and breast cancer (mortality ratio of 1.5).