The efficacy of bronchoalveolar lavage (BAL) in diagnosing pulmonary hemorrhage was studied in 51 immunosuppressed patients with new pulmonary infiltrates. Similar studies were performed in a control group of 8 nonimmunocompromised patients. Hemosiderin content in Prussian-blue-stained alveolar macrophages obtained by BAL was graded using a numerical scale. This "hemosiderin score" correlated closely with the degree of hemorrhage seen in corresponding histologic sections in the 26 patients from whom a lung biopsy or autopsy specimen was available. Severe pulmonary hemorrhage was ultimately diagnosed in 14 cases, and a mild degree of hemorrhage was found in an additional 19 cases. Thrombocytopenia and invasive fungal infections were statistically associated with severe hemorrhage, as was an increased percentage of alveolar macrophages in the BAL sample. This study demonstrates the efficacy of BAL in diagnosing occult pulmonary hemorrhage in the immunosuppressed host and highlights risk factors associated with hemorrhage in this setting.