A rare example of congenital radioulnar synostosis was observed in a sample of 160 male, female, and juvenile Lambayeque (Sicán) skeletons excavated from Huaca Cao Viejo, a mud brick platform mound built during the Early Intermediate Period and later utilized by a Late Intermediate Period Lambayeque population (ca. AD 950–1250) for interments. The affected individual was a young female, estimated to be 16–18 years of age at the time of death. To date, this is the first case of congenital radioulnar synostosis to be described from an archaeological context in South America, and one of 13 reported archaeological cases worldwide. Copyright © 2013 John Wiley & Sons, Ltd.