Mycobacterium avium-intracellulare complex (MAC) is a ubiquitous environmental microorganism whose pathogenicity ranges from innocuous colonization to disease, in immunocompetent as well as immunocompromised individuals. We sought to determine the clinical significance of MAC in sputum cultures of patients with pulmonary tuberculosis (TB). A retrospective analysis between January 1994 and March 1995 at Bellevue Hospital Center revealed both Mycobacterium tuberculosis and MAC in 35 patients (11% of all patients with TB). Of 27 patients reviewed, 52% were HIV-1 infected (median CD4 + 25 cells per microliter). Radiographic manifestations in patients with TB and MAC were similar to those seen in patients with TB alone. Both mycobacteria were cultured primarily from respiratory sources. M tuberculosis was usually cultured first or concurrent with MAC, and in nearly all cases, both species were recovered within 2 months of each other. Most patients improved clinically, bacteriologically, and radiographically with standard antituberculous therapy, except those with advanced AIDS, multidrug-resistant TB (MDR-TB), or disseminated MAC. We conclude that recovery of MAC in sputum is common in patients with pulmonary TB, regardless of HIV-1 infection, MDR-TB, or other clinical, bacteriologic, or radiographic attributes. MAC cultivation in most of these patients likely represents transient colonization, and in most cases is not clinically significant.