Among the patients who were examined with bone scintigraphy between April 1985 and March 1991, there were 27 patients whose initial clinical manifestation was bone metastasis and who were surveyed for the primary tumor site. The primary tumor site could be identified in 20 patients (74%), consisting of 9 patients with lung cancer, 3 with prostate cancer, 3 with hepatoma, 2 with renal cancer, and one each with thyroid cancer, adrenal cancer, and pleural malignant mesothelioma. In 17 of the 20 patients, the primary site had been detected within two months after presentation. Examinations which were helpful in identifying the primary site included chest radiography, sputum cytology, abdominal sonography, serum prostatic acid phosphatase level and pathologic examination of biopsy specimens. 99mTc-PMT scintigraphy was useful in the diagnosis of the hepatoma when accumulation was observed at the metastatic sites. In 2 patients, lung cancer had been recognized using follow-up chest radiography 3 and 6 months after presentation, respectively. One patient was diagnosed at autopsy as having adrenal cancer. In 7 patients the primary site remains unknown. Histology examination of the biopsy specimen performed in 6 of these patients revealed 4 to be adenocarcinoma and 2 undifferentiated carcinoma. The average survival period of the 17 patients who died was 9.5 months. Four patients are alive, and the outcome in the remaining 6 could not be determined.