In a population-based case-control study of the association between use of diuretics and renal cell cancer, 120 white men whose cancer was diagnosed during the years 1980-1991 and 86 white women diagnosed in 1960-1991 were identified among members of the Kaiser Permanente Northwest health plan. Controls were plan members who had been individually matched to cases on sex, race, age, and time period in the plan. Data on diuretic use and other selected variables were abstracted from outpatient and inpatient medical records. In an analysis confined to exposures present at least 2 years before case diagnosis, the odds ratios associated with any use of a diuretic drug were 2.2 (95% confidence interval 1.2-3.9) for men and 1.8 (95% confidence interval 1.01-3.2) for women. Increased duration of diuretic use was associated with an increased risk of renal cell cancer. This association was not restricted to one class of diuretic and was not confounded by cigarette smoking or body mass. Both hypertension and use of nondiuretic antihypertensive drugs were closely associated with diuretic use, and it was impossible to disentangle fully the effects of these three separate exposures. These results, together with those of prior studies, are generally compatible with the view that there is an association between diuretic use and the incidence of renal cell cancer, but the interpretation of that association remains in question.