Twenty hips in twenty patients had development of femoral endosteal cortical erosion after a cementless total hip replacement with the Harris-Galante porous-coated implant. Serial anteroposterior and lateral radiographs of all of the affected femora were reviewed to determine the interval between the operation and the first recognition of the osteolytic lesion or lesions, as well as to evaluate the radiographic appearance and progression of the lesions. The twenty affected hips were followed for an average of fifty-three months (range, twenty to seventy-seven months) after the operation. Osteolysis was first noted radiographically at twelve to sixty-six months (mean, thirty-nine months) postoperatively; in most patients, it occurred around the distal portion of the prosthetic stem. Twelve (60 per cent) of the femoral components were shown to be loose, as proved either radiographically or operatively. Three of these components had been judged radiographically to be stable when the endosteal erosion had first been identified, but they had subsequently migrated. The remaining eight hips had no evidence of loosening. The osteolysis was more severe in the femora in which the component was loose than in those in which it was stable. Of the fourteen patients who were followed, twelve (86 per cent) had an increase in the size of the osteolytic lesions with time, and only one patient had some radiographic evidence of healing.