The charts and radiographs of 79 patients with osteogenesis imperfecta (OI) actively followed at the authors' institution were reviewed to determine the incidence of acetabular protrusion (AP) in OI and to evaluate the clinical consequences of this pelvic deformity. All 79 patients had iliac crest bone biopsy and pelvic radiographs. They were subdivided into OI type 1, 3, and 4, according to the Sillence classification, and type 5, according to Glorieux. AP was measured on pelvic radiographs, using the center-edge angle of Wiberg and the acetabular ilioischial line. Twenty-six patients (33%) had radiologic evidence of AP, with the highest incidence in OI type 3 (69%) and OI type 5 (54%). Only 3 of these 24 patients (13%) had medical problems that could be related to AP: severe constipation and hip ankylosis. The authors conclude that in this series of 79 patients with OI, AP is not uncommon and in rare cases may lead to severe medical complications.