Fifty six consecutive, primary simultaneous bilateral total knee arthroplasties were prospectively compared to 172 consecutive, primary unilateral total knee arthroplasties. Outcome measures included length of stay, analgesic and blood requirement as well as range of movement and functional scores. All procedures were performed by a single surgeon using a uniform technique, including a postoperative flexion regime. Range of movement, analgesic requirement and complication rates were similar in each group. The bilateral cases stayed in hospital on average 1.7 days longer and required considerably more blood.