A 41/2-year-old girl with a sacral (pelvic) rib is reported. Three additional cases are reviewed from the literature. Histological, radiological and embryological evidence substantiate this "rib" anomaly, although coccygeal "duplication" rather than "rib" may be suggested by the presence of two bony elements separated by a "joint". All the affected cases were females, the "rib" being always right-sided, and possibly being a cause of feto-pelvic disproportion. Associated mild skeletal anomalies have occasionally been observed.