A clinical study was conducted to compare the amount and rate of movement and the tipping of canines retracted on 0.016 inch and 0.020 inch round wire with a continuous force of 200 grams and medium-width 0.022 inch by 0.028 inch nonangulated Siamese brackets. On one side canines were retracted on 0.016 inch wire and on the other side of the same arches, on 0.020 inch wire. Over 10 weeks, the mean amount of movement for twenty-one canines on the 0.016 inch wire was 3.37 mm., and for the twenty-one canines on the 0.020 inch wire it was 2.99 mm. The mean rate of movement in twenty-five arches was 1.37 mm. per month on the 0.016 inch wire and 1.20 mm. per month on the 0.020 inch wire. Over a period of 10 weeks, the mean amount of tipping for seventeen canines on the 0.016 inch wire was 5.3 degrees, and for the seventeen canines on the 0.020 inch wire it was 1.7 degrees. Since less tipping occurred on the 0.020 inch wire and the rates of movement were similar, there appears to be an advantage in retracting canines along 0.020 inch round wire rather than on 0.016 inch round wire. It would seem, also, that a greater force is not required to slide a tooth bonded or banded with an 0.022 by 0.028 inch bracket slot along an 0.020 inch round wire than along an 0.016 inch round wire as some laboratory studies suggest.