Summary— In a prospective study of 100 patients undergoing transurethral resection, changes in serum sodium were estimated during the per-operative and post-operative periods. The weight of prostate resected and the volume of irrigant fluid used influenced the changes in serum sodium. In 93 patients these changes were not statistically significant. In only seven patients were significant falls in serum sodium recorded, in one case to a level of 103 mmol/l. All seven exhibited the clinical features of the transurethral syndrome and there was one post-operative death which we attributed to it. The TUR syndrome undoubtedly exists and its incidence in this series was 7%. Its pathogenesis and clinical management are discussed.