Background: Malignant peripheral nerve sheath tumor (MPNST) is a heterogeneous group of neoplasms that shows divergent differentiation potential, and the cytological findings are largely nonspecific. There are no previous reports specifically alluding to the cytological findings of MPNST showing fibroblastic differentiation (fibroblastic MPNST). Case: A 59-year-old woman with neurofibromatosis type 1 developed a rapidly enlarging tumor on the scalp. A metastatic lesion in the ileum was examined cytologically. Irregularly shaped clusters of spindle cells with wavy or buckled nuclei and fibrillary cytoplasm were found as well as many similar isolated cells. Histopathologically, the tumor showed a spindle cell sarcomatous appearance typical of MPNST. Immunohistochemically, the tumor cells were negative for S-100 protein and epithelial membrane antigen but immunoreactive for vimentin, CD10 and CD34. Ultrastructurally, the tumor cells were not invested by the basal lamina and had abundant rough-surfaced endoplasmic reticulum and a subplasmalemmal accumulation of microfilaments with dense patches. These immunohistochemical and ultrastructural findings were consistent with fibroblastic or myofibroblastic differentiation. Conclusion: Although there are no cytological findings specific for fibroblastic MPNST, the wide spectrum of differentiation in MPNST should be kept in mind during cytological examination of soft tissue sarcomas.