Intravesical administration of Bacillus Calmette-Guerin (BCG) causes a localized cell-mediated immune response. The intensity of this inflammatory response may be gauged by measuring the levels of both interleukin-2 (IL-2) and an inhibitor of interleukin-2 (IL-2-IN) activity in the urine during the hours after a BCG instillation. The levels of urinary IL-2 and IL-2-IN in the sixth week of BCG therapy predicted the subsequent clinical course in a group of 25 patients (P less than 0.01). Measurement of urinary IL-2 and IL-2-IN activity may be used to identify accurately those patients likely to develop a tumor recurrence, thereby sparing them the risk associated with inadequately treated bladder cancer. Since IL-2 and IL-2-IN are competitive with respect to biologic activity, and since relatively high urinary levels of either IL-2 or IL-2-IN activity correlated with a favorable clinical course, the authors conclude that the presence of bioactive IL-2 in urine is not required for the prevention of recurrent superficial bladder cancer.