Postoperative chest radiographs on 100 children who had undergone cardiac operations were evaluated to determine the frequency of subpulmonary effusions after surgery. Of the 83 patients in whom adequate erect postoperative radiographs were available, 9 (11%) had effusions confirmed by lateral decubitus radiographs. On the frontal chest x ray film, the evidence of an effusion was an increase in distance between the diaphragm and air in the gastric fundus, or an apparent elevation of the right hemidiaphragm. None of the patients examined echocardiographically had associated pericardial effusions. When a subpulmonary effusion was detected diuretic treatment was started or continued. None of the patients had radiographic evidence of residual fluid when they were seen two weeks after their discharge from hospital.