PURPOSE: A safety study was conducted on a cohort of 25 patients who underwent lateral unicompartmental arthroplasty with a biconcave mobile-bearing insert. METHODS: The first 25 lateral mobile-bearing unicompartmental knee replacements, performed in a general hospital by one surgeon, were prospectively reviewed, with a minimum follow-up of 1 year. RESULTS: One bearing dislocation occurred 4 months postsurgery. The Oxford knee score improved in all patients from a preoperative mean of 23.3 (range 8-40, SD 8.4) to a postoperative mean of 42.1 (range 23-48, SD 6.7). General patient satisfaction at a mean follow-up of 20 months was excellent in 84 %, good in 12 % and fair in 4 %. The mechanical axis as a measure on full-leg standing radiographs improved from 5.7° valgus (range 1°-16°, SD 4.1°) to 1.7° valgus (7° to -3°, SD 2.1°). Mechanical alignment correction averaged 4.0° (range -1° to 15°, SD 3.9°). CONCLUSION: The mobile biconcave insert design in the lateral unicompartmental knee replacement seems appropriate as a innovative, anatomy imitating solution, resulting in a good clinical outcome. Still, bearing dislocation remains a concern, especially in extended indication. LEVEL OF EVIDENCE: Case series, Level IV.