Histiocytosis X is a disease of unknown etiology, characterized by a mass of proliferating histiocytes, plasma cells and inflammatory cells foaming a granuloma within the reticuloendothelial elements of any organ in the body. In the central nervous system (CNS), hypothalamic disorder of histiocytosis X is often found, but histiocytosis X in other regions is quite rare. We report a case of a 5-year-old girl with histiocytosis X of the zygoma presenting as a mass lesion in the tentorium cerebelli. A computed tomographic (CT) scan demonstrated a tumor at the left tentorial region, extending along the dura mater of the tentorium cerebelli. Magnetic resonance imaging (MRI) revealed a low signal intensity region on both T1 and T2-weighted images. MRI with Gd-DTPA showed a homogeneous enhanced mass extending to right and inferior sites with a thickened tentorium. As the thickened dura matter continued from the left middle fossa to the mass lesion, the tumor was considered to arise from the left zygoma and extend to the tentorium cerebelli. CNS extension of histiocytosis X is manifested either as (1) the cerebral type or (2) the dural type. Many cases of cerebral type histiocytosis X including hypothalamic disorder have been reported. Only 6 cases of the dural type of histiocytosis X have been described. Although the lesions of the cerebral type of histiocytosis X show prolonged T1 and T2 values on MRI, the MRI findings of the dural type have not been reported. The present case is the first report of the appearance of the lesion on MRI.(ABSTRACT TRUNCATED AT 250 WORDS)