In a prospective, randomized study, the effectiveness of chest physiotherapy (CPT) was evaluated in preventing postoperative atelectasis in children after heart surgery. Postoperative clinical variables and chest x-ray findings of atelectasis were compared in two groups: 19 patients receiving CPT and 25 patients not receiving CPT (NCPT). Chest physiotherapy was associated with significantly more frequent (p less than 0.01) and more severe (p less than 0.01) atelectasis than NCPT. Atelectasis was not significantly associated with temperature elevation, age, or presence of preoperative left-to-right shunt.