We reviewed two similar groups of patients with medial osteoarthritis of the knee treated by unicompartmental arthroplasty. The group receiving an Oxford meniscal-bearing implant, with no medial release, showed significantly better mechanical alignment than that receiving a fixed-bearing implant. Under-correction, with its ominous mechanical implications, was much more common with the fixed-bearing design. Over-correction was rare and was seen in both designs about equally. Degenerative stenosis of the intercondylar notch was common and appeared to put the anterior cruciate ligament at risk of rupture, especially after correction of the varus deformity. We consider that postoperative leg alignment and soft-tissue balance after unicompartmental knee replacement are determined more by the implant design and the surgical technique than by any variation in soft-tissue contracture. Release of the medial collateral ligament is not necessary for realignment, but a generous notchplasty is often needed to allow normal anterior cruciate ligament function.