Seventeen patients who developed urethral strictures following catheterisation have been studied; 14 of these (82%) were catheterised for standard monitoring purposes during and after major heart surgery, which was a coronary artery bypass graft (CABPG) in 12 cases. The possible aetiological mechanisms for the development of this complication are discussed. Animal experiments were designed and carried out to compare the effects of different catheter materials on the urethral mucosa. Silicone catheters provoked the least inflammation and rubber the most. A further experiment determined the flow rates through catheters of different sizes. From our results, we suggest that the careful insertion of a small silicone catheter, distally fixed, is least likely to induce stricture formation.