A few patients undergoing pelvic irradiation present with a prosthetic hip. The presence of this metal object often interferes with the lateral treatment fields if a four-field box irradiation technique is used. Measurements of the effect such a prosthesis may have on the dose delivered to the pelvis for beams of 60Co gamma rays and 10 MV and 25 MV X-rays, as a function of field size and depth have been made. The results show a strong variation in the dose behind the prosthesis depending on the position of the measurement, with little or no increase through the center of the head or ball, but a sharp drop either around its circumference or along the stem of the prosthesis. Measurements made with ionization chambers through the head of the prosthesis indicate an average dose decrease of approximately 2% for 25 MV X rays and average increases of approximately 2% for 10 MV X rays and 5% for 60Co. There is little variation with field size and depth except close to the prosthesis, at 25 MV. Measurements also indicate an increase in dose close to the upstream surface of the prosthesis at all energies, but in the downstream direction, only the data at 25 MV shows an increase.