Mycobacterium malmoense was isolated from pulmonary material from 4 patients. Two patients had repeatedly positive smears and cultures along with roentgenographic progression of pulmonary disease in the absence of another pathogen. These 2 patients therefore meet the criteria for diagnosis of pulmonary mycobacteriosis. Isolation of the organism may represent colonization in a third patient, and M. malmoense has been isolated from a fourth patient on 2 occasions. It is not yet definite, however, that the pulmonary process is due to mycobacterial disease. Although uncommon, pulmonary disease caused by this organism has been reported from Europe. Only 1 prior case of pulmonary disease caused by M. malmoense, however, has been reported in the United States.