A unique osseous tumor, which arose in the humerus of a 44-year-old white man, is reported. The lesion was designated atypical fibrous histiocytoma because of the absence of atypical mitoses despite prominent nuclear pleomorphism of tumor cells. The tumor was composed mainly of spindle cells, tightly packed in a storiform pattern. Clear histiocytes and tumor giant cells were occasionally seen. Toward the proximal and distal ends, the tumor showed increasing fibrogenesis, and the ends were composed of areas of packed clear histiocytes and acellular myxoid matrix with focal calcification. Ultrastructurally, five cell types comprised the tumor, but fibroblast-like cells predominated. Histiocyte-like, xanthomatous, giant and undifferentiated cells were observed only occasionally. The patient has been followed for five years after disarticulation without developing evidence of distant metastasis.