In 71 patients (41 men, 30 women) with advanced renal failure maintained on hemodialysis (HD), continuous ambulatory peritoneal dialysis (CAPD), or within 8 weeks after the commencement of dialysis therapy (ED), studies were performed searching for evidence of serositis and the blood levels of circulating immune complexes (CIC), measured by the polyethylene glycol precipitation method. Mean CIC levels were elevated in these patients compared to the 58 healthy subjects (mean ± SEM 725 ± 40, and 260 ± 34 µg/ml, respectively). Serositides were significantly more prevalent in ED patients compared to both remaining groups, and tended to occur less frequently in CAPD than in HD patients. CIC levels were significantly lower in CAPD than in ED patients. The levels of CIC were significantly higher in patients with serositis compared to those without evidence of serosal inflammation (mean ± SEM 868 ± 71 and 677 ± 53 µg/ml, respectively). In addition, immune complex-like materials were found in all of 4 effusate samples and in 10 of 27 peritoneal dialysates. It is suggested that immune complexes may be the ‘uremic toxins’ responsible for the serositis of renal failure, and may be secreted into serous effusions. Immune complex-like material can be removed in peritoneal dialysate, which may contribute to the lower CIC levels and lower incidence of serositis in the CA