A progressive study was carried out in 49 patients who underwent prostatectomy (10 transurethrally and 39 transvesically) to assess the effects of furosemide given intravenously for post-operative bladder irrigation. All 10 transurethral patients and 17 of the transvesical patients received furosemide, the remaining 22 patients receiving formal external bladder irrigation with saline. The results showed that post-operative infection was lowest and bed stay shortest in the patients with furosemide auto-irrigation. There was also less need for blood transfusion. No significant changes in haemoglobin and packed cell volume were observed in the transurethral patients; however, both decreased significantly in all 39 patients with transvesical prostatectomy, whether or not furosemide was used. Auto-irrigation with furosemide was found to be associated with a significant post-operative decrease in serum potassium but not in serum sodium levels. When external irrigation with saline was used, there was a significant increase in serum sodium.