A 50-yr-old man arrived at our department for a rehabilitation prescription after a right knee medial collateral ligament sprain. Magnetic resonance imaging showed medial collateral ligament partial rupture and medial femoral condyle increased signal intensity indicating bone edema. After treatment, the patient still complained of weight-bearing knee medial compartment pain. A lateral wedged insole was prescribed to decrease medial compartment compression forces. Initial response to insole use was good, but soon after, the patient complained of severe worsening knee pain. On examination, the lateral joint line and condyle palpation were tender. Insole use discontinuation was recommended, and another magnetic resonance imaging scan was performed. It showed an high T2-weighted signal intensity, representing bone marrow edema comprising a volume of 5 x 5 x 4.5 cm of lateral femoral condyle. Discontinuation of orthosis use relieved the pain, and the edema disappeared. To our knowledge, lateral femoral condyle painful bone marrow edema after lateral wedged insole use has not been previously described. The findings of this case report indicate that lateral wedged insole prescription should be carefully evaluated.