Thirty seven patients whose sputum cultures had yielded positive isolates of Mycobacterium malmoense during the years 1978-1983 have been reviewed. Significant pulmonary infection was present in 34 patients (92%), 3 of whom had only single isolates cultured from their sputum. The significance of isolates in the remaining 3 patients was not established. There was pre-existing pulmonary disease in 22 patients and another 4 were taking immuno-suppressive drugs. Various drug regimens were used to treat the condition but the best responses were seen in 5 patients (13.5%) who received 3 standard drugs given for between 18-24 months. Relapse occurred in 3 of another 5 who were treated with the same combination but given for less than 18 months. Omission of ethambutol from this standard regimen was associated with an unfavourable course in another 7 patients. Regimens which included the second line drugs ethionamide and cycloserine were given to 10 patients. The responses in this group were poor and were probably related to drug toxicity and poor patient compliance. Four of these patients eventually underwent successful resectional surgery.