This is a prospective study of 36 patients with lung infections caused by M. avium/M. intracellulare. The division of the patients into 2 prognostic groups ("good"/"others"), and the guidelines for their treatment are based on a previous report (3). The criteria for the division were co-existing lung disease and dyspnoea on admittance. The patients in group "good" were treated with isoniazid, rifampicin and ethambutol, in group "others" these were supplemented with one or more drugs. As in the previous study (3), the prognosis was far better for group "good" than for group "others". The patients in group "good" should be treated with isoniazid, rifampicin and ethambutol, unless the course of disease is unsatisfactory. The mortality was high in group "others" and half of the deaths were related to mycobacteriosis. Some patients became culture negative and others showed decreasing numbers of bacteria, but the value of the supplementary drugs could not be proven.