In a case-control study conducted in 1978 in Detroit, Michigan, as part of the National Bladder Cancer Study, the proportions of artificial sweetener users in a hospital and a population control series were compared. The study was based on interviews with 305 hospital controls and 440 population controls, as well as 391 patients with transitional or squamous cell carcinoma of the lower urinary tract. The proportion of artificial sweetener users among all hospital controls was higher than that among population controls. Among male hospital controls, it was found that 44% had ever used artificial sweeteners, compared to 38% of the male population controls. For females, the corresponding proportions of artificial sweetener users were 55% and 42%. Thus, relative risks estimated using all hospital controls were lower than relative risks estimated using population controls. When controls hospitalized for obesity-related diseases were excluded from the hospital control group, the proportion of artificial sweetener users and the relative risks for males were identical to those estimated with population controls (relative risk = 1.1). These results suggest that restriction of the control group to those patients hospitalized for non-obesity related diseases is a satisfactory procedure for selecting a control group in hospital-based studies of the effects of artificial sweeteners. For females, little or no change in the proportion of artificial sweetener users or in the relative risks was observed after exclusion of controls hospitalized for obesity-related diseases. However, the number of female subjects was small, and the results noted for females may have been due to chance.