Peritonitis during peritoneal dialysis is the most frequent complication associated with this dialysis technique. We studied patients undergoing peritoneal dialysis when they were without peritonitis and during episodes of clinical infection. Peritonitis was associated with a significantly decreased dialysate effluent volume, increased absorption of glucose, clearance of urea and creatinine, and protein loss in the dialysate effluent. We suggest that the changes occurring to the peritoneal dialyzing surface with peritonitis might be explained by alterations in peritoneal blood flow, effective membrane surface area, or permeability.