Furosemide remains the drug of choice for patients with the transurethral resection syndrome. Furosemide is often used intraoperatively to treat patients suspected of having excessive irrigant absorption. To examine the efficacy of such therapy, a randomized study was performed in which furosemide was administered to patients undergoing routine transurethral resection of the prostate (TURP) to determine the effect of furosemide on electrolyte and fluid volume conservation in these patients. Seven patients treated with furosemide on completion of TURP had a statistically significant delayed drop in serum sodium values after normal initial values (139 mEq/L to 134 mEq/L). Seven untreated control patients did not have a similar drop in sodium values, and the difference between groups was significant. We suggest that furosemide be used with caution in patients undergoing routine TURP, and when given it should be accompanied by the infusion of an isotonic balanced salt solution.