PURPOSE: Multiple bone metastases that appear on 99mTc-MDP bone scans tend to be distributed in the axialskeleton, including the proximal humeri and femora;this is similar to the distribution seen in Batson'sparavertebral plexus. The purpose of this study was, therefore, to evaluate- by examining the anatomicdistribution of metastases- the metastatic mechanism of cancer cells of various primary tumors in Batson's plexus. MATERIALS AND METHODS: Three hundred and forth-five known cases of primary tumors and multiple bone metastaseswere confirmed by bone scan imaging. The axial skeleton was divided into seven parts: skull, ribs, scapulas,spine, pelvis, proximal humeri, and proximal femora. In addition, the spine was divided into cervical, thoracicand lumbar areas. RESULTS: Among the 345 cases, bony metastases were distributed as follows: ribs, 186 (53.9%);spinal areas, 172 (49.9%); pelvis, 94 (27.2%); proximal femora, 85 (24.6%); skull, 63 (18.3%); proximal humeri, 45(13%); and scapulas, 31 (9%). Among the 243 primarily thoracic cancer cases (e.g. lung, breast or esophagus),distribution was as follows: ribs, 142 (58.4%); spinal areas 111 (45.7%); and pelvis, 54 (22.2%). Among the 69cancers which were primarily of the upper abdomen (e.g. stomach or liver), distribution spinal areas, 44 (63.8%);ribs, 33 (47.8%); and pelvis, 26 (37.7%). While the 33 primarily pelvic cancers (e.g. prostate, uterine cervix orbladder), were distributed between spinal areas (17, 51.5%), the pelvis (13, 39.4%), and the ribs (11, 33.3%).There were no statistically significant differences in the distribution of bony metastases according to primarycancers. CONCLUSION: Multiple bone metastases, as seen on bone scan images, coincided with the distribution ofBatson's plexus. We therefore conclude that although pelvic tumors can metastasize to the axial skeleton bydirectly anastomosing into Batson's plexus, other primary tumors metastasize from systemic veins to Batson'splexus as a result of the occurrence of venous reflux when intrathoracic or intra-abdominal pressure is increased.