We report on a monozygotic twin with sirenomelia and anencephaly. This association seems to have been observed only twice before. In addition to these anomalies the patient had cleft palate, rachischisis, and segmentation vertebral anomalies affecting the cervical and the upper thoracic spine. The second twin was a liveborn female infant with a large cystic paraovarian teratoma, and duplication of internal genitalia.