Data on 28 patients with malignant histiocytosis (MH), fourteen patients with virus-associated hemophagocytic syndrome (VAHS) and two patients with familial erythrophagocytic lymphohistiocytosis (FEL) were collected from 21 hospitals in Japan to study the serum ferritin levels and clinical features. At diagnosis, the serum ferritin values were a median of 3,000 ng/ml (range, 59-270,000 ng/ml) in MH and 10,500 ng/ml (range, 44-68,600 ng/ml) in VAHS/FEL. Clinical signs and symptoms were not substantially different between MH and VAHS/FEL. Thus, serum ferritin markedly increased in the majority of MH/VAHS/FEL patients and should be a useful marker of disease activity in either neoplastic or reactive histiocytic proliferative disorders.