The most important factors for long-term survival of cementless prostheses are the initial fixation, the osteoconducting properties of the metal shell and the bony response. Porous tantalum, a new biomaterial with a geometric structure similar to trabecular bone, was reported in animal studies to allow for bone ingrowth even when 3-mm gaps exist between the implant and the bone. This new material may improve the durability and stability of hip arthroplasties. We analysed the behaviour of the underlying acetabular bone, based on radiographs taken 46 months or longer after implantation of monoblock cementless acetabular cups made of porous tantalum. Clinical evaluation was done by means of the Harris Hip Score. The acetabular ARA-score, ranging from poor to excellent, was excellent in 80 % of the cases, 46 months or longer after implantation. The clinical condition of the patients as assessed with the Harris Hip Score was excellent for 65 % of the patients.