Clinical and laboratory features have been reviewed in 66 episodes of disseminated histoplasmosis that occurred during two large urban outbreaks in Indianapolis. Immunosuppression, age greater than 54 years, and presence of other serious underlying illnesses predisposed to the disseminated form of the disease; only 21% of patients lacked one of these risk factors. Central nervous system findings, splenomegaly, hepatomegaly, and lymphopenia suggested disseminated disease but were present in only about one-third of patients. Miliary or diffuse pulmonary infiltrates also suggested dissemination and were noted in about one-third of patients, while mediastinal lymphadenopathy was present in only 17%. Histoplasmal serologic tests, positive in 90% of patients, provided useful diagnostic clues. The diagnosis could be confirmed by culture in 88% of patients, and special stains were positive in about two-thirds. Although 10% of patients recovered without treatment, 11 patients (17%) died because of failure to suspect the diagnosis and initiate therapy promptly. Amphotericin B was effective in all patients receiving at least 500 mg, but relapse occurred if the total dose was less than 30 mg/kg. Ketoconazole appeared effective in non-immunosuppressed patients but not in those with underlying immunosuppression; however, a controlled trial comparing ketoconazole and amphotericin B is required to establish the role of this new fungistatic oral agent.