A review of clinical records and radiographs revealed that five of 10 patients with conjoined twinning (three of six ischiopagus and two of two pyopagus twins) had congenital vertebral anomalies including hemivertebrae (3), multiple thoracic anomalies (1), right hemisacral agenesis (1), and three lumbar vertebra (1). No mirror image anomalies were identified, and different regions of the spine were involved in the two cases in which both twins had anomalies. Coexisting visceral and musculoskeletal anomalies included dextrocardia (3), atrial septal defect (1), congenital vertical talus (2), Sprengel's deformity (1), and multiple unilateral foot anomalies (1). Hip subluxation or dislocation was seen in five of eight ischiopagus or pyopagus twins after separation and required femoral and pelvic osteotomies. Lumbar scoliosis not associated with congenital vertebral anomalies was seen in two ischiopagus twins. These observations suggest an association between ischiopagus and pyopagus conjoined twins and congenital vertebral anomalies, and their coexistence is explained best by a nonspecific teratogenic insult during early embryogenesis.