The purpose of this study was to evaluate whether immunologic status could predict the development of peritonitis in children on continuous ambulatory peritoneal dialysis (CAPD). Twenty-one patients (12 boys, 9 girls) aged 10.1 +/- 4.32 years (range: 23 months to 14 years) were studied. The mean duration of CAPD treatment was 12.88 +/- 6.69 months (range: 2-22 months). Twelve healthy children (mean age 11.5 years) were selected as a control group. Lymphocyte subpopulations (CD3, CD4, CD8, CD19, NK, and IL-2R) were determined by double-color flow cytometry (Becton-Dickinson). Statistical evaluation was made by Student's t-test. CD4 (38.4 +/- 7.1% vs. 28.0 +/- 5.4%), CD4/CD8 ratio (1.30 +/- 0.4 vs. 0.96 +/- 0.3), and B lymphocyte (19.9 +/- 8.9% vs. 12.0 +/- 3.2%) levels were significantly higher in CAPD patients compared with controls. CAPD patients showed significantly lower natural killer (NK) cell values than controls (12.7 +/- 7.0% vs. 27.3 +/- 8.3%). Apart from CD19 values (21.9 +/- 10.4% vs. 12.0 +/- 3.2%) there were no significant differences between CAPD patients without infection and the control group in the laboratory parameters studied. On the other hand, CD3 (66.3 +/- 7.9% vs. 55.9 +/- 9.7%), CD4 (38.4 +/- 7.1% vs. 28.0 +/- 5.4%), and CD19 (6.3 +/- 3.3% vs. 12.0 +/- 3.2%) levels were significantly higher in CAPD patients with infection compared with the controls. CAPD patients with infection showed significantly lower NK activity (12.7 +/- 7.1% vs. 27.3 +/- 8.3%) than those in the control group. In conclusion, these results can explain the increased vulnerability to peritonitis in CAPD patients compared with healthy subjects. Additionally, immunologic status can predict the development of peritonitis in children treated with CAPD.