Seventy-five patients with pulmonary infection caused by Mycobacterium avium-intracellulare were studied to determine whether results of therapy correlated with in vitro susceptibility testing of mycobacterial isolates. Fifty patients responded to therapy and 25 were nonresponders. The total number of drugs received by responders did not differ significantly from the total number of drugs received by nonresponders. However, responders received significantly more drugs to which their isolate was susceptible in vitro than did nonresponders (2.4 +/- 1.2 versus 1.4 +/- 1.0, p less than 0.001). We conclude that patients with pulmonary M. avium-intracellulare infection should receive chemotherapeutic agents to which their isolate is susceptible in vitro.