The purpose of this study was to investigate the risk of transitional cell carcinoma among subjects with an intake of acetaminophen, aspirin, some other drugs and with some intercurrent diseases. The source person-time ('study base') included subjects living in Stockholm in 1985-1987. The study included 325 subjects with a transitional cell carcinoma of the urinary tract and 393 controls randomly selected from the source person-time. Data were obtained by a postal questionnaire supplemented by a telephone interview. A relative risk (with a 95% confidence interval) of 1.6 (1.1-2.3) was obtained after an intake of acetaminophen, adjusted for age, aspirin, gender and smoking. Conversely, a 30% decrease in risk was obtained after an intake of aspirin. No details in the exposure substantiated the finding for acetaminophen. The inherent validity problems of observational studies, and the weak evidence in this and previous studies of the association between acetaminophen and transitional cell carcinoma, makes available epidemiological evidence insufficient to regulate the use of this commonly ingested analgesic. Increased risks were, in addition, found for tetracyclines, nitrofurantoin and a history of allergic asthma and a decreased risk found for rheumatic symptoms. The findings stress the nonepidemiological data concerning the potential carcinogenicity of acetaminophen and may be a foundation for future research of some other drugs and diseases.