Various authors have found peritonitis rates in older peritoneal dialysis (PD) patients to be higher, similar, or even lower when compared to rates in younger populations. We prospectively analyzed all episodes of peritonitis registered in the 381 patients (219 males, 162 females; mean age, 55.5 +/- 17.0 years) who were treated with PD during four years (1993-1996) in our multicenter group. Patients were distributed into two groups. Group A included 138 patients aged 65 years or over (mean age: 72.3 +/- 5.1 years); group B included 243 patients aged below 65 years (mean age: 46.0 +/- 13.6 years). No differences were seen in general PD characteristics. Normalized protein catabolic rate (nPCR) was higher in younger patients (p = 0.001). Patients in group A experienced more peritonitis (135 episodes; 1 episode per 16.7 patient-months) than patients in group B (198 episodes; 1 episode per 21.7 patient-months; p = 0.01). Although no differences were seen in the general characteristics of the peritonitis episodes, gram-negative peritonitis and peritonitis not achieving a cure were more commonly secondary to enteric bacteria in group A (p = 0.03). We conclude that PD patients aged 65 years or over are at higher risk of peritonitis. Also, in this age group, gram-negative peritonitis and peritonitis with any evolution except cure are more likely to be due to enteric bacteria.