The incidence of complications after pulmonary resection was studied in 122 patients with different pulmonary diseases and varying degrees of ventilatory insufficiency. These patients had preoperative tests to determine if there was a contraindication to the type of surgery planned. From the results of the pulmonary function tests a resection of more than a lobe was considered contraindicated if the vital capacity in the first second was less than one liter and if the ventilation was evenly distributed between the two lungs. Because of this, surgery could not be carried out in nine patients. The pulmonary resection was uneventful in 97 patients, 25 patients (22 % of the total number) had complications and 5 of them died (4 % mortality rate). The pulmonary function studies showed a marked ventilatory insufficiency and a functionless left lung, in the patient who died from respiratory insufficiency after a left pneumonectomy. In the other patients mechanical and infectious factors were involved in the production of complications. Pulmonary function studies were needed to establish the contraindications for surgery. Regional lung function studies were valuable to map the extent of resection.