Histologic changes in the acetabuli of adult dogs were studied following total hip replacement with current cementing techniques. Active bone remodeling was evident throughout the acetabulum by 2 weeks after operation and continued throughout the 6 months of the study, leading to considerable hypertrophy of the trabecular bone of the acetabulum. By 6 months after operation, a thick (average, 0.4 mm), fibrous membrane had formed at the bone-cement interface in 83 of 84 sections examined. Direct apposition of bone to cement was seen only within the 2-mm fixation holes. Formation of a fibrous membrane appears to be intrinsic to the cementing of the acetabular component, perhaps because geometry of the acetabulum precludes effective pressurization of the cement.