Little data are available about the characteristics, outcome, and risk factors for polymicrobial peritonitis in patients on continuous peritoneal dialysis. We therefore reviewed the 43 episodes of polymicrobial peritonitis that occurred in 39 of our patients over the 11.5 years of our program. Polymicrobial peritonitis represented 9% (43/492) of all peritonitis episodes. Only three episodes of polymicrobial peritonitis were associated with an enteric source. Sixteen percent (7/43) of the polymicrobial peritonitis episodes were associated with catheter infections. Staphylococcus aureus was one of the cultured organisms in 33% of the polymicrobial peritonitis episodes. Patients with polymicrobial peritonitis had higher rates of tunnel infection (0.28/yr v 0.18/yr) and overall peritonitis (1.23/yr v 0.81/yr) than patients with single-organism peritonitis. Black patients were more likely to have polymicrobial peritonitis (12/56 v 27/338 whites, chi 2 = 9.8, P less than 0.005). Patient age, gender, time on peritoneal dialysis, insulin dependence, and cause of end-stage renal disease had no influence on polymicrobial peritonitis. Significantly more catheters were removed for polymicrobial peritonitis than for single-organism peritonitis (17/42 v 89/420, chi 2 = 7.05, P less than 0.01), but in 60% of the episodes, the polymicrobial peritonitis was successfully treated without catheter removal. Polymicrobial peritonitis often occurs without gram-negative organisms and extension of catheter infection may be involved in some cases. For unclear reasons, black patients are at higher risk of polymicrobial peritonitis.