Total hip arthroplasty was performed in 19 patients using the lateral approach. Laser Doppler flowmetry was used to measure the blood flow in the anterior and posterior parts of the gluteus medius. The anterior part was split and detached. The posterior part was left intact. The blood flow in the gluteus medius of the anterior part was decreased by 15.4%, while that of the posterior part remained constant during surgery. Postoperatively, the Trendelenburg sign remained positive in 3 of 16 patients. No correlations were observed between a positive Trendelenburg sign and reduction in blood flow of the gluteus medius.