Two cases of osteomyelitic lesions, histologically characterized by prominent foam macrophages arranged in a mosaic pattern, had the following pathologic features: neutrophil infiltration, foam cell formation, numerous plasma cells, and fibrin precipitates. According to similar situations in other organs, the term xanthogranulomatous osteomyelitis was applied to these cases. This differential diagnosis can be applied to other bone lesions in which foam macrophages may play an important role.