As part of a prospective cohort study of 388 men undergoing transurethral resection of the prostate (TURP) for benign prostatic hypertrophy, pre-operative factors and the outcome of surgery during the first post-operative year were compared between patients in whom their surgeon felt the principal reason for operating was chronic retention (37%), acute retention (with no chronic retention) (28%), or symptomatic prostatism (with no history of chronic or acute retention) (35%). Although in many respects the patients in the 3 diagnostic categories were similar, patients with chronic retention were more likely to be younger, of higher social class and to have worse general health. Patients with acute retention were more likely to present with a urinary tract infection and to have electrocardiographic abnormalities, and symptomatic patients presented with more severe urinary symptoms. Minor differences between the categories with regard to post-operative morbidity and mortality were not statistically significant at the 5% level. However, some significant differences did exist. Patients with acute retention were more likely to experience urinary and non-urinary infections and impotence after surgery, while symptomatic patients reported less improvement in their health status as regards pain and social isolation. These results suggest that the method of categorisation is clinically valid and a necessary distinction to make when auditing TURP.