In order to aid patient selection we have analysed the radiological progression of arthritis in the retained compartments following lateral unicompartmental knee replacement (UKR). Patients undergoing lateral UKR (St Georg Sled, Waldemar Link) between 1988 and 1999 were assessed. Radiographs taken post-operatively within 8 weeks and at 5 years were assessed. OA was classified using both the Altman and Ahlbach scoring systems. Identifying information on each radiograph was obscured so that the observer was blinded. Each radiograph was reviewed twice to assess intra-observer variability. Patients were assessed clinically using the Bristol Knee Score (BKS) at 1 and 5 years post-operation. Reproducibility for the Ahlbach systems was shown to be very good (kappa = 0.86) and the Altman moderate (kappa = 0.41). Thirty-two lateral UKRs were assessed at 5 years. Six out of 32 of the knees assessed showed definite progression of OA on the Ahlbach score in the retained medial compartment and 11 out of 32 on the Altman. There was a statistically significant increase in the grade of OA, as assessed by both systems, at 5 years as compared to the post-operative radiograph (p <0.001). The definite progression group had a mean BKS 10 points lower at 5 years. Six patients required revision to TKR for progression of arthritis in the retained compartments. Lateral compartment UKR can be a very successful procedure, but there is a greater rate of progression of arthritis in the retained medial compartment than following medial UKR.