Twenty-one patients with Marfan's syndrome participated in a study to assess the incidence of and radiographic measurements significant for protrusio acetabuli. Our data show that the incidence of protrusio acetabuli in Marfan's syndrome is 31%. The most sensitive radiographic parameter to determine protrusio acetabuli is crossing of the acetabular line by the iliopectineal line. The presence of protrusio in Marfan's syndrome was not related to the bone mineral content of the hip and pelvis. Protrusio acetabuli in Marfan syndrome's also did not correlate with clinical symptoms. Based on our results, the presence of protrusio acetabuli alone is not an indication for early surgical intervention.