Atelectasis may be defined as collapse of the lung due to absence of air within the alveoli. It may involve anatomic segments, lobes, or whole lungs but also may be a diffuse miliary process, as in the adult respiratory distress syndrome. The key to treatment are the anticipation and prevention of atelectasis in various clinical situations, the recognition and treatment of underlying disease, and the prompt initiation of vigorous treatment once atelectasis is found. Repeated assessment by physical examination is necessary to determine the presence of atelectasis and its response to treatment. During the period of January, 1981 to October, 1990, 100 patients with atelectasis were treated in the department of Thoracic and Cardiovascular Surgery, Pusan National University Hospital. There were 70 males and 30 females ranging from 3 days to 79 years of age. The occurrence ratio of right to left side was 2.1 : 1. The underlying pathologic lesions of atelectasis were pneumonia with effusion(28), lung ca.(24), pulmonary tuberculosis(24), and chronic empyema(9), The treatment procedure for atelectasis were closed thoracostomy in 26 cases, ressection in 21 cases, therapeutic bronchoscopy in 14 cases and etc.