We assessed changes in proximal femur diameter during 5-8 years follow-up of 20 clinically successful press-fit cementless isoelastic implants. Total bone diameter, medullary diameter and cortical thickness were measured over the distal half of the prosthesis and just beyond the prosthesis tip. During the follow-up period total bone diameter did not change. However, cortical thickness declined by an average of 1 mm, while the size of the medullary canal increased by the same amount. As the patients remained asymptomatic during this period, the phenomenon observed did not appear to be related to loosening. We concluded that age-related changes in bone diameter can lead to widening of the tight interface between the stem and the femur.