In a review of 25 patie-nts with pulmonary infection caused by unclassified mycobacteria the course of the disease is related to the method of treatment. Five patients were followed while not being treated; in four the disease progressed and in one it remained static, and in each case the organism persisted in the sputum. Sixteen patients were given combinations of streptomycin, isoniazid, and para-aminosalicylic acid for periods which varied between three months and two and a half years; there was bacteriological sputum conversion in six, although in each case the organism showed in vitro resistance to these drugs. Eleven patients were treated primarily with combinations of other anti-tuberculosis drugs for at least 12 consecutive weeks, after which the mycobacteria could no longer be cultured from the sputum in eight, in each case the organism showing in vitro sensitivity to the drugs. Ten patients received combined medical and surgical treatment, which was successful in eight and a failure in two, one of whom died post-operatively. A favourable response occurred most frequently when the disease was confined to one pulmonary radiographic zone and when less than 10 positive sputum cultures had been obtained. These results show that in this type of disease the response to chemotherapy is uncertain, in vitro drug sensitivity tests are of limited value, and combined medical and surgical treatment is usually effective.