Histoplasmosis is a fungus disease of nearly worldwide distribution. Because of its wide distribution and tendency to be endemic near large cities, it is the most common systemic mycosis in the United States, with an estimated 500,000 new infections per year. Although millions of persons in the United States have been infected with this fungus, few show signs of disease. Histoplasmosis has emerged as an important opportunistic infection in immunocompromised patients, including those with AIDS. This review of histoplasmosis builds upon basic descriptions found in standard texts, incorporating important recent observations.