Abstract This study was conducted to assess the symptomatic effects of the insole with an elevation of 12 mm composed of urethane (urethane insole) or of sponge rubber (rubber insole) with subtalar strapping in patients with medial compartment osteoarthritis of the knee (knee OA). The setting was an outpatient clinic. Eighty-four patients with knee OA were prospectively randomized, and evaluated and treated with the urethane or rubber insole for 4 weeks. Randomization was performed according to birth date and each participant was categorized into the urethane group or the rubber group. The percentage of remission of Lequesne index of severity for knee OA was compared between urethane and rubber insole groups at the conclusion of the study. Participants were asked to report adverse effects of use of the respective insoles. The percentage of remission was significantly improved in the urethane insole group (n = 42) compared with the rubber insole group (n = 42) (P = 0.001). Adverse effects were more common in the rubber insole group (17 out of 42, 40.5%) than in the urethane insole group (8 out of 42, 19.0%), and this was statistically significant (P = 0.028). The inserted insole in combination with subtalar strapping had a more natural form-fit to the sole than the insole insert alone. In our study of the subtalar strapping insole, an insole composed of urethane, was more comfortable than that of rubber sponge.