In the past, medial osteoarthritis (OA) knee with symptomatic patellofemoral (PF) arthritis has not been recommended for a unicompartmental knee arthroplasty (UKA). However, recent studies have reported that UKA has shown good results in patients with medial OA of the knee, including those with PF arthritis. The purpose of this study is to compare the results between patients with medial OA knees; those with severe arthritis of the lateral facet of the patella and patients without severe arthritis of the lateral facet of the patella following mobile bearing UKA. We have prospectively evaluated 104 patients (114 knees) who had undergone an Oxford mobile bearing UKA. The mean follow-up was 19.05 months (range 12.30–29.70 months). The patients were divided into two groups: group I consisted of eighty patients (88 knees) who did not have severe arthritis of the lateral facet (Outerbridge grade 0–2) and group II had twenty-four patients (26 knees) who had severe arthritis of the lateral facet (Outerbridge grade 3, 4). We recorded the incidence of anterior knee pain, knee scores, pain scores, and functional scores in comparison of the two groups. The visual analog scale (VAS) and incidence of post-operative anterior knee pain had not shown any significant differences. The VAS for post-operative anterior knee pain was 0.11 (SD 0.56, range 0–3 point) versus 0.12 (SD 0.59, range 0–3 point) for group I and group II patients, respectively (P = 0.98). The incidence of post-operative anterior knee pain was 4.5 versus 3.8 % for group I and group II patients, respectively (P = 0.88). The pain scores and functional scores had not exhibited any differences. However, the knee scores of patients with severe arthritis of the lateral facet of the patella was worse than those seen in patients without severe arthritis of the lateral facet of the patella with a statistical significance. It was scored as 96.78 (SD 4.56, range 85–100) versus 94.43 (SD 4.50, range 81–100) for group I and group II patients, respectively (P = 0.02). Anterior knee pain, pain scores, and functional scores were not different between the two groups following a medial Oxford UKA. However, the knee scores of patients with severe arthritis of the lateral facet were worse than those in patients without severe arthritis of the lateral facet of the patella.