Involvement of the skeletal system by congenital syphilis is well documented in the literature, chiefly in the form of radiologic studies, including periostitis, osteitis, and osteochondritis. Because congenital syphilis is generally recognized clinically, tissue biopsy is virtually never performed. Therefore, the histopathologic findings are less well documented and mostly exist in the older literature. We report herein the clinicoradiologic and pathologic features of a 2-month-old infant who initially presented with absence of left arm movement. Radiographs of the left humerus revealed a mid diaphyseal cortical irregularity/lytic lesion and periosteal reaction. Follow-up skeletal survey showed similar findings in other extremity long bones. A bone biopsy of the humeral lesion revealed a destructive fibrohistiocytic process composed of a sheet-like proliferation of epithelioid to spindled histiocytes, without obvious granulomas, accompanied by occasional lymphocytes and neutrophils with rare plasma cells. Immunohistochemical stains showed diffuse positivity for CD31, CD68, and S-100, but CD1a was negative. Initially, the case was interpreted as "atypical fibrohistiocytic proliferation," favoring Langerhans cell histiocytosis. A few days later the results of serologic testing revealed a rapid plasma reagin of 1:256. Immunostaining for Treponema pallidum on the initial biopsy confirmed the presence of innumerable spirochetes, with a predilection for blood vessels. The patient was treated with a 10-day course of intravenous penicillin with complete resolution of the bone lesions and resulting symptomatology. To our knowledge, the above pathologic features of congenital syphilis of bone, especially in regards to its mimicry of childhood histiocytoses, have not been previously reported.