The prevalence, severity, and clinical importance of acetabular erosion secondary to hemiarthroplasty of the hip are largely unknown. Of a series of 241 patients (258 hips), sixty-nine patients who were treated with primary cemented Thompson hemiarthroplasty for seventy-two subcapital fractures of the hip were personally interviewed and examined, and radiographs of the hip were made, three to fourteen years after the procedure. The factors that had the highest correlation with the severity of the erosion were the level of physical activity and the duration of follow-up. Erosion through acetabular cartilage into bone developed in thirty-four of the thirty-eight hips in active patients and in none of the thirty-four hips in inactive patients (p less than 0.001). The severity of the erosion increased with time, but only in the hips in active patients, and then it was associated with pain and disability during walking. The patient's age and type of residence (private home or nursing facility) at the time of the fracture were the best predictors of the postoperative level of activity. Patients who were older than seventy years and from nursing homes and those who were older than eighty years and from private homes generally continued to be inactive; in these two groups of patients, the long-term results were good. Of the forty-nine hips in patients who were less than eighty years old and from private homes, thirty-four were in patients who continued to be active.(ABSTRACT TRUNCATED AT 250 WORDS)