The absorption of large volumes of irrigation fluid is a major problem in transurethral prostatic surgery (TUR-P). Various indicators have been tested to monitor fluid absorption with regard to continuous registration and sufficient accuracy. The volumetric fluid balance is not suitable as a routine method because of its inaccuracy. Easily accessible parameters are unspecific because of surgical bleeding (haematocrit [Hct]), or are interfered with by physiological counter-regulatory actions (serum sodium [Na] concentration). In 1986 Hulten et al. suggested adding 2% ethanol to the irrigation fluid as a marker and investigated it intermittently in the expired air with an alcohol-test appliance. In a prospective clinical study of 17 patients undergoing TUR-P under spinal anaesthesia, expiratory concentrations of alcohol that was added to the irrigation fluid (2% ethanol in Purisole, Fresenius, Bad Homburg) were monitored. Gas was continuously sampled from the nasopharynx through a nasal cannula and the ethanol concentration was measured using a modified diverting anaesthetic gas monitor (Normac, Datex, Helsinki) that allows continuous as well as early detection of the absorbed irrigation fluid with reliable accuracy for clinical use. In addition, at intervals of 10 minutes we measured blood alcohol, end-tidal alcohol (Alcotest 7110, Dräger, Lübeck), haematocrit, serum Na concentration, and blood gases. Sixty-eight measurements were obtained from the 17 patients. As shown in other studies, serum Na (r2 = 0.68) and Hct (r2 = 0.39) correlated poorly with the irrigation fluid as determined by serum alcohol levels. In contrast, the expiratory alcohol measurements with the Alcotest 7110 (r2 = 0.93) and Normac devices (r2 = 0.85) were closely related. Continuous monitoring of the expiratory alcohol concentration with a Normac monitor closely reflects blood alcohol concentration, and may hence serve as a useful semiquantitative monitor of irrigation fluid absorption during TUR-P.