The incidence of radiographically visible ectopic ossification around the hip joint was redorded in 145 hip joints of 132 patients treated by endoprosthetic replacement. There were 56 total hip replacements with a McKee-Farrar and 39 with a Brunswik type of endoprosthesis; in a further 50 hips the femoral head was replaced with a Moore or Thompson prosthesis. An identical antero-lateral surgical approach was used in all, without detachment of the greater torchanter. The patients were re-examined 3, 6, and12 months after the operation. The extent of ectopic ossification was graded from 0 to III and correlated with pain and with the function and mobility of the operated hip. Ectopic ossification of varied extent was recorded in 37 % after total hip replacement and in 38 % after replacement of the femoral head. Of these ossifications 95 % were recognizable within 3 months; they did not increase in size, but often in density during the following months. One case of bony ankylosis was noted. Ectopic ossification of slight (grade I) to moderate (grade II) degree did not cause pain or affect the function of the operated hip, but reduced the mobility of the affected hip. The difference in mobility between grade I and grade II was significant (p less than 0.05), and between grade 0 and grade II highly significant (p less than 0.001).