An osteogenic sarcoma arose in the right orbit of a 7-year-old boy some 5 years after the right orbit had been treated by four courses of radiotherapy (total dose approximately 13,000 rads) for a multicentric retinoblastoma. Death occurred 6 months after the orbital tumor was first detected. Study of the orbital tumor by electron microscopy revealed a cell population of varied morphology in which two main types were identified. In one group, the cells were large with radiolucent cytoplasm, which contained long branching segments of rough endoplasmic reticulum. In the second group, the cells were smaller with irregular nuclei and an electron-dense cytoplasm, which contained short segments of dilated rough endoplasmic reticulum and numerous mitochondria. The first group of cells closely resembled osteoblasts, while the second group had some features of osteoclasts or their percursors. The branching processes of the tumor cells were separated by an amorphous ground substance, which contained collagen-like fibrils and hydroxyapatite crystals. Crystal deposition was in some instances in close relation to extracellular membrane-bound vesicles.