BACKGROUND: Unicompartmental knee arthroplasty (UKA) is an effective treatment for unicompartmental end-stage knee arthritis. Simultaneous bilateral UKAs for patients with bilateral knee arthritis can reduce costs, number of anesthetics, and overall rehabilitation time. It is unknown how the long-term outcomes of unilateral and simultaneous bilateral UKAs compare. METHODS: In total, 1,939 unilateral and 1,939 simultaneous bilateral medial mobile-bearing UKAs (n = 3,878) from the National Joint Registry were propensity score matched. Kaplan-Meier and Cox regression were used to compare implant survival, revision indications, and mortality. RESULTS: The 10-year implant survival in the simultaneous bilateral group was 92% (95% confidence interval [CI] 90-94) and in the unilateral group was 90% (95% CI 88-92). The simultaneous bilateral group had a lower revision risk (hazard ratio [HR] 0.73, p = 0.01). Revisions for pain were lower in the bilateral group (0.5% vs. 1.2%, p = 0.01). There were no differences in patient mortality. Subgroup analyses found similar trends in 10-year implant survival and revision risk with both cementless (simultaneous bilateral 98% CI 95-99; unilateral 95% CI 91-98; HR 0.66, p = 0.27) and cemented fixation (simultaneous bilateral 91% CI 89-93; unilateral 90% CI 88%-92%; HR 0.85, p = 0.28). CONCLUSIONS: Simultaneous bilateral UKAs had better 10-year implant survival and similar mortality to compared with single-unilateral UKAs. For patients with severe symptomatic bilateral unicompartmental knee osteoarthritis, simultaneous bilateral UKAs could be considered to be a safe and effective procedure, particularly as only one operation and postoperative recovery is required. LEVEL OF EVIDENCE: Level IV. See Instructions for Authors for a complete description of levels of evidence.