Intra-abdominal pressure (IAP), forced vital capacity (FVC), and forced expiratory volume at 1 sec (FEV1) were measured in 18 stable continuous ambulatory peritoneal dialysis (CAPD) patients maintained on 2-liter exchanges, in the supine, sitting, and upright positions after infusing dialysis solutions in 0.5-liter increments up to 4 liters as tolerated. Thereafter, five patients did not increase to 3-liter volumes (RUT-0), four used 3-liter volumes occasionally (RUT-1), and nine chose 3-liter volumes for routine dialysis (RUT-2). IAP was similar in all groups and dependent on the intraperitoneal volume (IPV). The mean IAP increased 2.0, 2.7, and 2.8 cm H2O/liter of IPV in the supine, upright, and sitting positions, respectively. The patients of the RUT-0 group had dramatic deterioration (up to 42%) of FVC and FEV1 in the supine position with IPV above 2 liters. The patients with the greatest deterioration of pulmonary functions could not continue the measurements above 3 liters of IPV. Two of these patients were switched to 1-liter overnight exchanges. Even in patients who tolerated up to 4 liters, FVC and FEV1 decreased significantly in the supine and sitting positions, with IPV greater than 3 or 4 liters, respectively. In the upright position, the values did not decrease significantly below those with the empty abdomen up to 4.6 liters of IPV. Each liter of IPV increased the abdominal girth by 2.1 cm. Exchange volume and frequency should be individualized. In our studies, 50% of the patients could increase daily dialysate volume from 8 to 9 liters while decreasing daily exchanges from 4 to 3.