Extreme obesity and leanness are risk factors for many types of cancer. An earlier American Cancer Society study (1959-1972) found a nearly twofold increased risk for death from all causes in men and women who weighed 40% or more above average for their age and height, and found elevated cancer rates as well. A new (1982), ongoing ACS prospective study of 1.2 million men and women continues to find increased death rates from all causes and from cancer in the very heavy and the very lean. Artificial sweetener (AS) use is an important correlate of relative weight in this population. The relationship between weight change during the year preceding enrollment and AS usage was studied in a highly homogeneous subgroup of 78694 women ages 50-69 years. The percentage of users increased with body mass index (BMI) and was inversely related to age. Users were significantly more likely than non-users to gain weight, regardless of initial BMI. Among those who gained weight, the average number of lbs gained by AS users was higher (by 0.5-1.5 lb) than the amount gained by non-users. Within the entire cohort, AS users of both sexes ate chicken, fish and vegetables significantly more often than did non-users and consumed beef, butter, white bread, potatoes, ice cream and chocolate significantly less often, suggesting that our weight change results are not explicable by differences in food consumption patterns.