OBJECTIVES: To investigate if absorption of irrigation fluid occurs during bipolar plasma vaporisation (BPV) of the prostate. To examine the clinical predictors of increased risk of fluid absorption and to assess if changes in serum electrolytes, venous pH, haemoglobin or haematocrit are able to detect intra-operative fluid absorption. PATIENTS AND METHODS: Over a 15-month period, 55 consecutive patients undergoing BPV of the prostate were investigated. The volume of intra-operative fluid absorption was measured using expired-breath ethanol measurements. Intra-operative irrigation was performed with isotonic saline containing 1% ethanol. The breath ethanol concentration was measured every 10 min during the operation and the volume of irrigation fluid absorption was calculated from these concentrations. Data on clinical (age, prostate volume, smoking status) and surgical variables (operation time, irrigation volume, appearance of capsular perforation) as well as intra-operative changes in serum electrolytes, venous pH, haemoglobin and haematocrit were recorded. RESULTS: The median (range) age of the patients was 67 (48-87) years and the median (range) prostate volume was 41 (17-111) mL. Nine patients (16%) showed a positive ethanol breath test during the procedure. The median (range) calculated fluid absorption in these patients was 346 (138-2166) mL. Three patients had a fluid absorption >500 mL. One patient with absorption of >2 L showed clinical symptoms (dyspnoea and agitation) during the operation under spinal anaesthesia. In the group of patients with fluid absorption, capsular perforation or injury to larger vessels was more often detectable. In the group of patients with fluid absorption, only venous pH showed a significant change during the operation (from median 7.41 to median 7.34, P = 0.02). The pH decrease was significantly greater in the fluid absorption group than in the group of patients without fluid absorption (0.09 vs. 0.02, P = 0.005). CONCLUSION: We have demonstrated that significant intra-operative fluid absorption can occur during BPV of the prostate. Care must be taken if using this procedure in patients with significant cardiovascular comorbidities. Respecting the anatomical borders of the prostate seems to play a relevant role in preventing fluid absorption during the procedure. Venous pH could be used to detect potentially dangerous fluid absorption if intra-operative monitoring with breath ethanol measurements is not available.