The risk for kidney cancer was examined in a Danish cohort of 192,133 people on a hospital discharge register who had been given a diagnosis of hypertension, heart failure, or edema, and were presumed to be probable users of diuretics. The subjects were identified from 1977 to 1987 and followed-up for cancer through 1987. A total of 10,630 cancers was observed. While the risk for all cancers was increased slightly (standard mortality ratio [SMR] = 122, 95 percent confidence interval [CI] = 120-124), the risk for renal cell carcinoma was more than doubled (SMR(men) = 221, CI = 192-253; SMR(women) = 246, CI = 213-283). Increased risks were found in all age groups, and, although surveillance bias was present initially, the risk increased consistently in the years following discharge. Risk estimates for individuals discharged with hypertension were similar to those for the total cohort. Use of diuretics was validated in a random sample of 100 individuals. More than 70 percent were taking diuretics at the time of discharge. The increased risk for renal cell carcinoma in this cohort may indicate either that diuretics are involved in the etiology of renal cell carcinoma or that the risk can be attributed to confounders, including smoking, which affect risk for both the discharge diagnosis and renal cell carcinoma.