Developmental dysplasia of the hip (DDH) is a spectrum of disease starting in childhood and in many cases persisting into adulthood. The spectrum ranges from acetabular dysplasia, through hip subluxation to dislocation. The aim of this research was to determine the prevalence and pathoanatomy of acetabular dysplasia and subluxation in excavated human skeletal remains, to complement past research on dislocation in DDH. The material under study was the medieval cemetery of St. Mary Spital in London, in use from c.1100 to 1539 AD. A series of 572 adults with both hips preserved were analyzed. Acetabular dysplasia was indicated by a shallow acetabulum with upward sloping roof. Subluxation was suggested by degenerative change along the margin of the acetabulum suggestive of labral tears, and degenerative change in the outer part of the acetabular roof suggestive of osteoarthritis. The prevalence of DDH (acetabular dysplasia, subluxation, or dislocation) was 1.7%. Because this a congenital musculoskeletal disorder of relatively high frequency, with significant variation in prevalence between populations around the world, it is a topic that warrants targeted research from physical anthropologists studying past populations. Am J Phys Anthropol, 2011. © 2010 Wiley-Liss, Inc.