Spirometry was carried out before and after operation in groups of patients undergoing three preselected surgical procedures. The patterns of impairment of pulmonary function in each group are discussed. Only in patients undergoing inguinal herniorrhaphy was spirometry of value in determining an 'at-risk' group for the development of respiratory complications. Discrimination was optimal on the 1st postoperative day, making it possible for 70% of patients to be discharged from hospital 48 h after this operation with little risk of subsequent respiratory complications.