Tuberculous psoas muscle abscess following chemoprophylaxis with isoniazid in a patient with human immunodeficiency virus infection. A 34-year-old man with human immunodeficiency virus infection and disseminated Mycobacterium avium and Mycobacterium intracellulare infection developed a right psoas muscle abscess due to Mycobacterium tuberculosis. The abscess occurred 18 months after completion of a 12-month course of chemoprophylaxis with isoniazid that was given because of a positive reaction with purified protein derivative of tuberculin. The adjacent vertebrae did not appear to be involved. The abscess was drained with a percutaneously inserted catheter, and he received standard antituberculous chemotherapy. Three weeks into therapy, a second drainage with a catheter was required. The isolation of two mycobacteria in this patient and the apparent failure of chemoprophylaxis with isoniazid are noted. 