We conducted a multicenter case-control study to identify risk factors for histoplasmosis among persons with acquired immunodeficiency syndrome (AIDS) and to evaluate predictors of a poor outcome (defined as death or admission to the intensive care unit). Patients with histoplasmosis were each matched by age, sex, and CD4 lymphocyte count to 3 controls. From 1996 through 1999, 92 case patients and 252 controls were enrolled. Of the case patients, 81 (89%) were men, 50 (55%) were black, 78 (85%) had a CD4 lymphocyte count of <100 cells/µL, 80 (87%) were hospitalized, and 11 (12%) died. Multivariable analysis found that receipt of antiretroviral therapy and of triazole drugs were independently associated with a decreased risk of histoplasmosis. Chronic medical conditions and a history of infections with herpes simplex virus were associated with poor outcome. Triazoles should be considered for chemoprophylaxis for persons with AIDS, especially those who take part in high-risk activities that involve frequent exposure to soil, who have CD4 lymphocyte counts of <100 cells/µL, and who live in areas where histoplasmosis is endemic.