Sixty-three patients completed medical treatment for lung abscess. Forty-three patients were, at the completion of medical treatment, without evidence of active infection and had a normal plain roentgenogram or a residual cyst-like cavity of less than 2 cm in diameter. Thirty-nine patients were examined 1 to 12 years later and only one had developed complications. Of six patients failing to meet these criteria for successful treatment five subsequently developed recurrence of infection or hemoptysis. Tomograms or bronchograms or both in 30 patients showed residual cystic or bronchiectatic changes in 17 but these patients had no related symptoms. There was a highly significant correlation between an unsatisfactory outcome and both cavity size and duration of symptoms prior to treatment. Neither location of the abscess nor age of the patient significantly altered the outcome.