In our previous report, we demonstrated normal cartilage and bone matrix formation and a defect of bone mineralization in hypophosphatemic (Hyp) mice using an ectopic bone formation system. That system consisted of an osteogenic sarcoma-derived bone-inducing substance. In this report, we describe the effect of phosphorus supplementation on abnormal bone mineralization. The osteogenic sarcoma-derived bone-inducing substance was implanted in Hyp mice or control mice. The Hyp mice were divided into two groups after implantation. One group was fed a normal laboratory chow, while the other was fed a high-phosphorus diet for 4 weeks of the experimental period. Normal control mice were fed the normal laboratory chow. The mean serum phosphorus level in the high-phosphorus diet group was normal at 2, 3 and 4 weeks after implantation. Using the method of 85Sr incorporation, the high-phosphorus diet group showed marked improvement in bone mineralization at 2 and 4 weeks after implantation, but incomplete improvement at 3 weeks. On the other hand, histological study of the high-phosphorus diet group at 4 weeks after implantation still showed a meaningful amount of the osteoid matrix formation compared to the control. These findings suggest that the abnormal bone mineralization in Hyp mice was mainly due to their abnormally low serum phosphorus level. However, still other abnormalities might exist and these might be responsible for the incomplete improvement in bone mineralization.