It has recently been shown that human alveolar macrophages can be selectively activated without systemic effect by the use of aerosolized interferon-gamma (IFN gamma), a cytokine that enhances macrophage oxidative and antimicrobial activity. We report the case of a 38-yr-old man negative for human immunodeficiency virus (HIV), with silicosis and advanced cavitary lung disease due to Mycobacterium avium intracellulare (MAI), who failed to improve despite 3 yr of continuous medical therapy with three or more drugs. He received three courses of aerosolized IFN gamma (500 micrograms 3 d per week for 5 wk in two courses and 200 micrograms 3 d a week for 5 wk after a short single trial of subcutaneous IFN gamma). The numbers of MAI decreased in the sputum during therapy, but cultures of the organism remained positive at the same level for the first two treatment periods. The patients sputum became AFB smear negative and the number of colonies decreased significantly after the third course of IFN gamma therapy. Cessation of IFN gamma was associated with a rapid increase in the numbers of MAI in the sputum. Aerosolized IFN gamma can be considered as an adjuvant to conventional drug therapy, with a good tolerance, in cases of lung disease caused by resistant MAI.