Primary bone neoplasms developed in beagle dogs briefly exposed by inhalation to aerosols of 238PuO2. 238PuO2 was initially deposited in the respiratory tract where it was retained with a half time greater than 100 days. A portion of the 238Pu was solubilized and translocated to the liver and skeleton. Five years after exposure, 46 osteosarcomas developed in 35 of 144 exposed dogs. The cumulative absorbed radiation doses to skeleton for these dogs ranged from 210 to 830 rad. Of the 46 bone tumors, 22 originated in the vertebrae, 12 in the humeri, 6 in the pelves, and 6 in miscellaneous long and flat bones. Most of the tumors were well-differentiated sarcomas. Only 10 of the tumors metastasized; the lung was the organ most often invaded. Bone tumors were associated with lesions of radiation osteodysplasia. The number of bone tumors found in this study indicated that inhaled 238PuO2 was an effective skeletal carcinogen. The rate of solubilization in the lung and translocation to bone may be a factor in the radiation dose pattern and type and location of bone tumors that developed after inhalation of 238PuO2.