We conducted a prospective, randomised study on primary total knee replacements to evaluate the effects of tourniquet use on total calculated blood loss using Gross formula, post-operative measured blood loss, operating time, need for blood transfusion, post-operative pain, analgesia requirement and knee flexion. Forty patients were operated on with the use of an arterial tourniquet with pressure of 350 mmHg (group A), and 40 patients without the use of a tourniquet (group B). Total calculated blood loss was significantly increased ( P=0.0165) without the use of a tourniquet. There was no significant difference in measured blood loss or operating time. The median units of blood given were similar in both groups. In spite of autologous transfusions 14% of patients received additional homologous transfusions. At 6 h post-operatively pain was significantly less ( P=0.0458) in group B but was similar at 24 and 48 h. There was no significant difference in analgesia requirement. The mean change in total flexion in group B was significantly better ( P<0.001) at 5 days than in group A, but knee flexion was similar at 10 days and 3 months. Knee arthroplasty operations without the use of a tourniquet cause a greater blood loss but have only small benefits in the early post-operative period.