In five straight-stemmed, proximally porous-coated femoral components that were retrieved at revision arthroplasty from patients who had radiographic and clinical evidence of loosening, there was growth of bone into the porous coating. The components had been inserted during a primary arthroplasty in one woman and four men. The patients ranged in age from thirty-seven to sixty-seven years. Three patients were heavy, and all five were active. All patients had had an excellent early result from the initial arthroplasty; at the one-year follow-up, the mean Harris hip score had been 91 points. Pain in the hip developed in all of the patients, between one and three years after the index procedure. Initial radiographs had revealed excellent position and fixation of the prosthetic components, but the components then subsided between one and three and one-half years after the index procedure. All of the femoral components were found to be grossly loose at the revision operation. Nevertheless, all of the prosthetic components demonstrated growth of bone into 4 to 44 per cent (mean, 24 per cent) of the pore spaces available for ingrowth. Woven bone and fracture callus were found in the curettings from the proximal part of the femur. The findings in these five patients suggest that late failure of uncemented porous-surfaced femoral components can occur despite the presence of extensive ingrowth of bone. These failures may be the result of fatigue fractures of the trabeculae of the osseous ingrowth into the porous surfaces. Caution is warranted in the liberal use of these prosthetic components in heavy, active patients.