This study was designed to investigate the humoral immune status and immunological abnormalities in relation to the pathogenesis of peritonitis in patients undergoing continuous ambulatory peritoneal dialysis (CAPD). The levels of serum and dialysate total protein, immunoglobulins (IgG, I-A, I-M, and IgG subgroups), and C3 were measured at baseline and at the end of the study in 12 children and 10 adult patients on CAPD. The relationship between the levels of immunoglobulin and C3 and the frequency of peritonitis was investigated. Peritonitis occurred at a frequency of one episode every 3.9 and 6.3 patient-months in children at the beginning and at the end of the study, respectively; however, the respective rates for adult patients were one episode every 5.6 and 9.8 patient-months. The levels of serum immunoglobulin and C3 were normal in adults at the beginning and at the end of the study. However, the levels of serum IgG were low in 4 of 12 pediatric patients. Of the 4 patients with low total IgG, I-A was low in I and IgM was low in 2 patients. Of 4 patients with low total IgG, 2 also had low IgG1 and IgG2. However, 3 of the patients with normal IgG had low IgG2 levels at the end of the study. The levels of C3 were low in 6 of 12 and in 3 of 10 pediatric patients at the beginning and at the end of the study, respectively. The protein levels of dialysate were increased at 1-year follow-up in both groups, but the difference was only statistically significant in adult patients (P<0.05). No correlation was found between the levels of serum and dialysate immunoglobulin and the frequency of peritonitis in either group. In this study, hypogammaglobulinemia and hypocomplementemia were determined in pediatric patients undergoing CAPD. Although these immunological deficits are not major factors in the occurrence of CAPD-related peritonitis, they may contribute to the frequency of peritonitis.