Objective This study describes a modified 4-hour peritoneal equilibration test (PET) for analyzing peritoneal transport characteristics of proteins with different molecular weights and predicting daily peritoneal protein losses in children on chronic peritoneal dialysis (PD). Design Cross-sectional study. Setting A single regional pediatric dialysis unit in a teaching hospital. Patients 9 stable pediatric dialysis patients; 4 were on continuous ambulatory PD, 5 were on continuous cycling PD. Main Outcome Measures Serum and dialysate concentrations of IgG, albumin, β2-microglobulin, and transferrin were determined during a PET. Changes in dialysate-to-plasma (D/P) ratios were determined hourly. Agreement between PET-derived and measured daily peritoneal protein losses was examined. Results The D/P ratio decreased with increased molecular radius ( p < 0.0001). Many children had low plasma levels of IgG, albumin, and transferrin, but elevated levels of β2-microglobulin. The D/P ratio increased linearly during the PET for all measured proteins, regardless of molecular weight. There was close correlation between 4-hour PET protein losses and 24-hour losses during routine PD. Conclusions Proteins are lost through the peritoneum according to their size, demonstrating linear transport kinetics during a 4-hour PET. The PET-derived data predicted daily protein losses in children on chronic PD. This approach might help to eliminate inaccuracies due to incomplete dialysate collection.