Functional residual capacity (FRC) of the lung was measured by the closed-circuit helium equilibration method before and for five days after upper abdominal operations in 28 patients (25 had cholecystectomies). Measurements in many were made with the patient both sitting in bed and sitting in a chair. Vital capacity (VC), residual volume (RV), and forced expiratory volume in one second (FEV1), as well as FRC, all decreased after operation, with the maximum decrease on days 1 and 2 and a gradual return toward preoperative values by day 5. Patients with 40% or less decrease in FRC after operation did not develop pulmonary complications. Change in position from bed to chair increased FRC 14.2% preoperatively and 17% postoperatively. The use of intermittent positive pressure breathing had no measurable effect on FRC. Similar changes in FEV1, VC, and RV also occurred, with maximum decreases on day 1.