A bloodless field is essential for orthopedic surgery. Although a touriquet is necessary, it is dangerous because of soft tissue damage. We studied the soft tissue pressure beneath a tourniquet in the thigh to identify the accurate pressure for the bloodless field, and recovery time of normal blood flow after tourniquet release as related to the time of tourniquet use. From June, 1986, to Marth 1987, the soft tissue pressure of 21 lower extremities beneath the tourniquets was measured by the slit catheter method, and the noraml blood flow recovery time of 21 lower extremities after the tourniquet release was measured by a photoplethysmograph. The results were follows : Tourniquet pressure of 400 mmHg was enough for a bloodless field. So we can prevent the tissue damage due to excessive tourniquet pressure. The soft tissue pressure beneath a tourniquet was not affected by the age, weight, height or thigh circumference of the subjects. The recovery time to normal blood flow after tourniquet relase was prolonged with the time of tourniquet use. The recovery time to normal blood flow after tourniquet relase was not affected by age, weight, height, or thigh circumference of the subjects.