The authors describe a population-based case-control study of cancer of the urinary bladder in New York State women aged 20-49 years (173 matched pairs) in 1975-1980. Both known and suspected risk factors for bladder cancer were examined for these rare, early-onset female cases. The odds ratio (OR) for cigarette smoking was 2.4 (95% confidence interval (CI) = 1.5-4.0). Cases consumed more coffee per day than did controls (p less than 0.05). More cases than controls reported heavy use of drugs that contained phenacetin (OR = 6.5, 95% CI = 1.5-59.2). In addition, more cases than controls reported that they had had a thyroid uptake procedure with radioactive iodine (iodine-131) (OR = 3.7, 95% CI = 1.4-11.0). The odds ratios for these factors remained elevated after simultaneous adjustment with conditional logistic regression models. Consumption of artificial sweeteners, occupational history, bladder or kidney cancer in a first-degree relative, and a history of bladder infections were examined and found not to be risk factors for bladder cancer in this study.