Many surgeons have sought to avoid valgus postoperative limb alignment when performing medial unicondylar arthroplasty under concern that such posture accelerates lateral tibiofemoral compartment joint space narrowing. To test whether postoperative limb alignment was associated with lateral compartment narrowing, we measured the lateral compartment joint space width from the most recent single-leg erect anteroposterior radiograph and similarly-obtained early postoperative radiographs of 113 medial unicompartmental arthroplasties (eight designs, 91 patients) that had a minimum of 10 years (range 10-19 years) clinical followup. The mean (+/- standard deviation) narrowing was 0.03 +/- 0.13 mm per year (< 0.10 mm per year, 89 knees) or 7% +/- 30% (< 25%, 90 knees). Narrowing increased with more valgus (less varus) of the early postoperative hip-knee-ankle angle. However, the effect of alignment was small (narrowing increased by 0.01 mm per year per each 1 degree) and it was not linked with an outcome of 25% or greater, 50% or greater, or 75% or greater narrowing. In summary, narrowing increased slightly with more valgus (less varus) early postoperative limb alignment, but limb alignment was a poor predictor of which knees would develop more complete joint space thinning. LEVEL OF EVIDENCE: Level IV, prognostic study. See the Guidelines for Authors for a complete description of levels of evidence.