Osseointegration of titanium implants in total hip arthroplasty. Osseointegration is defined as direct contact on the light microscopic level between living bone tissue and the implant. Using titanium screw dental implants in the jaw, a lasting interface under loaded conditions extending over a 20-year follow-up period has been demonstrated. This demonstration brings up the question whether a similar interface can be achieved in total hip arthroplasty (THA) between living bone and a titanium alloy implant under necessitated conditions of immediate loading. Two series of cases are reported. The first series used a femoral, press-fit, titanium alloy component and the second used a press-fit titanium acetabular component and redesigned femoral, press-fit, titanium alloy component. Both demonstrated a high percentage of good to excellent results. Roentgenograms showed that the geometrical changes in the redesigned femoral component gave early indications of a better fixation with loading in valgus, less subsidence, and less evidence of distal stress transfer. A two-and-one-half-year postoperative anatomic specimen study confirmed osseointegration to the press-fit titanium alloy femoral component. Multiple areas of contact between bone and metal without fibrous interposition were seen. Examination by electron microscopy supported the light microscopic findings. These findings support further use of smooth, press-fit titanium components in THAs without the need for porous coating, mesh, or other surface modifications. 