Recent papers have shown that malignant histiocytosis (MH) can be well controlled if the disease is diagnosed early and treated intensely. Previous reports proposed the usefulness of lymph nodes and liver biopsy or bone marrow aspiration in making a diagnosis of MH, but even when a combination of these three procedures was used, between 24 and 61% of patients with MH could not be correctly diagnosed. In the present study, clinical and autopsy findings in 25 patients with MH were reviewed to identify possible alternative diagnostic sites to lymph nodes, liver or bone marrow. Clinical symptoms were observed in the respiratory (40%) and gastro-intestinal (48%) tracts. Proteinuria was also frequently identified (60%). Autopsy findings showed that involvement of lung, stomach and small intestine, and kidney by atypical histiocytes were present in 100% of patients with respiratory symptoms, 85% of those with abdominal symptoms and 80% of those with proteinuria, respectively. From the analysis of this pattern of histiocytic infiltration, lung, renal, gastric or jejunal biopsies are demonstrated as sites for the diagnosis of MH.