A female infant weighing 3,800g was born at 41 weeks of gestation in a satisfactory cource to a healthy 23-year-old primipara. The infant had a parasite conjoined with her sacrococcygeal region. The autosite had no other anomalies. The asymmetrical conjoined twins were easily separated by operation 36 hours after birth. The parasite was 730g in weight. It had a femur, a tibia, a fibula, seven phalanger and hypoplastic pelvis. Vertebral structures were not present. There was no connection of viscerae between the parasite and the autosite. It had well develoed intestines, multilocular cyst, unilocular cyst, and lower limb. Histlogically, the wall of multilocular cyst consists of tissues of three germ layers that has differentiated to the various degrees. It was diagnosed, there-fore, as the Sacral Parasite. After the operation, her growth and development are favorable. Reviewing the Japanese literatures, the authors could find only 2 cases of sacrococcygeal parasite. The pathogenesis and diagnotic criteria of this anomaly were discussed.
