To evaluate the usefulness of a tracer of 1% ethanol in 1. 5% glycine in the early detection of irrigation fluid absorption during transurethral resection of the prostate (TURP). Patients (120) undergoing TURP were irrigated with 1% ethanol in 1.5% glycine solution and their expired air tested for alcohol every 10 min during the procedure. In all, 112 patients were assessed; over half of the patients absorbed the irrigation fluid and they had a significantly lower postoperative serum sodium concentration (P < 0.002). Fourteen patients (12.5%) absorbed over 500 mL and two (1.8%) developed clinical features of the TUR syndrome. The experience of the surgeon, the weight of resected chips and the operative duration were not significantly predictive of absorption. A tracer amount of ethanol in the irrigant is reliable for detecting absorption. Irrigating fluid absorption was unpredictable, thus supporting the case for routine monitoring.