BACKGROUND: The absorption of irrigation fluid during transurethral resection of the prostate (TURP) is determined primarily by hydrostatic pressure in the bladder and prostatic venous pressure. In comparison to spontaneously breathing patients, patients undergoing mechanical ventilation with positive pressure have a raised central venous pressure and a reduced venous return, both of which can influence intravascular absorption. The purpose of the prospective study was to compare the effects of general (GA) and spinal anaesthetic (SA) techniques on the perioperative absorption of irrigating fluid in patients undergoing TURP. METHODS: Forty patients undergoing TURP were randomised and assigned either to group GA or SA. Irrigating fluid absorption was traced by adding 1.5% (w/v) ethanol to the irrigating fluid. Perioperative blood ethanol concentration (BEC), haemoglobin concentration, haematocrit, serum sodium concentration and central venous pressure (CVP) were measured at 10-min intervals during TURP and at 30-min intervals while patients were recovering. Absorption routes were indexed by the BEC and changes in serum sodium concentrations. Where the BEC was greater than 0.05 mg.mL-1, absorption of irrigating fluid was assumed. For assessing the volume of irrigating fluid absorbed, the maximum BEC, the absorption rate, the area under the BEC curve (AUC), and the volumes calculated according to the Hahn nomogram (Volin) for each patient were taken into consideration. RESULTS: There were 15 cases of irrigating fluid absorption in patients receiving GA (75%), and 11 in those receiving SA (55%). CVP was significantly lower in spontaneously breathing patients with SA as compared to those with GA (P < 0.05). In patients with irrigating fluid absorption the maximum BEC (P < 0.02), as well as the rate of irrigant fluid absorption (P < 0.01), were significantly higher amongst patients receiving SA. In this group, the calculated area under the curve and the absorbed fluid volumes determined with the nomogram were significantly increased (P < 0.05). CONCLUSION: The absorption of irrigation fluid during the TURP is significantly more marked amongst spontaneously breathing patients with regional anaesthesia in comparison to patients undergoing general anaesthesia with positive pressure ventilation. The markedly lower central venous pressure before the start of irrigation should be considered as a possible cause of this effect.