Initial studies indicate that rifampin may be useful for the treatment of Staphylococcus aureus infections. Because bacterial resistance to rifampin may develop rapidly, its widespread use could result in the emergence of a resistant flora. This study evaluates the effectiveness of rifampin in reducing the nasal carriage of S. aureus and the rate at which resistant mutants emerge in a tuberculosis hospital where the drug was widely used. Anterior nares cultures were performed four times over a 13-month period. Carriage rates of S. aureus were 1.7% in 227 patients receiving rifampin, 7.8% in 190 patients receiving other antituberculous therapy, and 14.2% in 98 hospital employees (rifampin-treated versus other patients, P < 0.003; rifampin-treated versus employees, P < 0.001; employees versus other patients, P = 0.157). All four strains of S. aureus isolated from patients on rifampin therapy were rifampin resistant. All 16 strains isolated from patients not on rifampin and 15 of 16 strains isolated from hospital personnel were susceptible. One instance of apparent spread of a rifampin-resistant organism occurred in a hospital attendant who had never received rifampin.