We report a prospective study of the factors associated with acute urinary retention after total hip replacement in 103 consecutive male patients. Eleven patients (10.7%) developed retention after operation. Of the factors investigated before operation three had predictive value: inability to pass urine into a bottle whilst lying in bed, urinary peak-flow rates indicative of obstruction, and a history of previous bladder outflow problems. This study suggests that patients showing one or more of these factors should be assessed and if necessary treated by a urologist before arthroplasty, so as to avoid the need for catheterisation, and the consequent risk of deep infection.