The biology of skeletal metastasis is poorly understood. In order to establish an animal model of bone metastasis, cells from a human prostate cancer cell line (PC-3) were injected into the tail veins of athymic nude mice while the inferior vena cava was occluded. This technique was used in order to divert cells into the vertebral venous plexus. A control group of animals received tumor cells without caval occlusion. Bone lesions developed in 3/16 (19 per cent) experimental mice and in none of the control mice. The incidence of lung metastasis was significantly decreased in the experimental mice (5/16) as compared with non-occluded control mice (14/16). Two tumor sublines were established from explant cultures of bone lesions. Injection of these cells resulted in bone metastasis in 19/36 (53 per cent) mice (P = 0.03 compared with the parent line). The incidence of lung lesions was also increased. The predominant site of bone metastasis was the lumbar vertebrae; other affected sites were the pelvis and femurs. All bone lesions resulted in extensive bone destruction. The successful development of bone metastasis using the technique of caval occlusion lends support to the hypothesis that entry of cells into the vertebral circulation is an important step in the development of these lesions. This model should be of value in understanding the pathogenesis of bone metastasis, and in studying the effects of various agents on the prevention and control of these lesions.