By standardizing the diagnosis of primary tumors in cases of bone metastatic cancer of unknown origin and evaluating the diagnostic strategy, we provide a reasonable plan for finding the primary tumors of bone metastatic cancer. From June 2003 to December 2006, a total of 125 cases of bone metastatic cancer of unknown origin with no previous history of malignant tumor were admitted. The sites of bone metastases at the initial diagnosis mainly included the spine in 57 cases, the sacrum in 24 cases, the pelvis in 16 cases, the femur in 15 cases and the humerus in 7 cases. The primary tumors were diagnosed by routine physical examination, laboratory tests, chest radiographs and/or CT, abdominal and pelvic ultrasound and/or CT, and pathological biopsy. ? Primary tumors were found in 70.4% of the cases, from 40% of the lungs, 8% of the kidneys, 5.6% of the prostate, 4.8% of the liver, 4% of the breast, 2.4% of the thyroid and 2.4% of the stomach, and 3.2% of the others. Among the various examination methods, only 9.6% of the primary lesions were detected by physical examination, and 43.2% of the cases had tumor-specific antigen abnormalities. 60% of the cases were found to have primary tumors by imaging examination. Pathologic examination results were clear or suggestive of diagnosis in 66.4% of the cases. The 1-year survival rate of cases with primary tumor was 58.57%; the 1-year survival rate of cases without primary tumor was 22.93%. ? Most of the cases of bone metastatic cancer without a clear history of malignant tumor can be diagnosed with primary tumor after standardized examination, and the survival time can be prolonged after receiving appropriate treatment.