Most physicians fail to recognize Mycobacterium avium-intracellulare (MAI) as a major pathogen for pulmonary disease among patients admitted to hospitals throughout the United States. In a review of all records of positive MAI cultures during the 10 years beginning July 1, 1979, at The Mount Sinai Hospital, New York City, we have identified 244 patients who had pulmonary disease primarily or secondarily complicated by MAI. We also identified another 243 patients as false positive for MAI infection. We classed as false positives patients who had no subsequent positive culture and whose clinical picture was and remained incompatible with MAI infection. We identified four distinct clinical patterns in the 244 patients with true positive MAI infections: (a) pulmonary nodules ("tuberculomas") indistinguishable from pulmonary neoplasms (78 patients); (b) chronic bronchitis or bronchiectasis with sputum repeatedly positive for MAI or granulomas on biopsy (58 patients, virtually all older white women); (c) cavitary lung disease and scattered pulmonary nodules mimicking M. tuberculosis infection (12 patients); (d) diffuse pulmonary infiltrations in immunocompromised hosts, primarily patients with AIDS (96 patients). The diagnosis should be established either by surgical resection and culture of resected nodules, or by three repeated positive acid-fast bacillus cultures of sputum or fluid and tissue obtained by bronchoscopy, or by biopsy of other tissue which shows granulomas and one or more positive MAI cultures. Surgical resection is the best treatment for "solitary" MAI nodules. Multiple antituberculous drug therapy is indicated for patients with chronic infection that impairs function or causes hemoptysis. The presence of MAI in the sputum or lung aspirates of patients with AIDS usually heralds the presence of a preterminal disseminated infection.