Indwelling urinary catheter may induce an inflammatory reaction or even stricture of the urethra. Catheter encrustation and urinary infection are other disadvantages associated with long-term catheterisation. In the present study, 77 male patients were catheterised randomly as part of their normal treatment with 1 of 3 different types of catheter: 22 siliconised latex, 28 hydrogel-coated latex and 27 full silicone catheters. The mean duration of catheterisation was 2.2 days. The urethral inflammatory reaction was assessed from cytological urethral swab specimens. Catheter encrustation was studied using scanning electron microscopic (SEM) analysis. The full silicone catheters induced the mildest degree of inflammation in the urethra, the percentage mean of inflammatory cells in smears being 20%. In both latex catheter groups the value was 36%. Neither the age of the patients nor the duration of catheterisation had any effect on the inflammatory reaction, which was more marked in patients with haemodynamic abnormalities. The hydrogel coating effectively prevented encrustation, while siliconised latex catheters were the least resistant to encrustation. The inflammatory reaction was variable in all patients. The use of urethral catheters should be restricted and suprapubic tubes should be used instead, particularly in patients with shock-like circulatory changes. By developing the biocompatibility and physical properties of urinary catheters, more compatible devices may be manufactured.