This longitudinal investigation examined the shift from use of a marking time to an alternating stepping pattern by young children. A set of twin males was videotaped between ages 37 and 46 months climbing stairs of 3.8-17.8 cm height. One boy began to alternate consistently on the highest steps at 41 months, the other at 46 months. Anthropometries (leg lengths) and a measure of foot overshoot (maximum height of the foot over the stair) were used to investigate the timing of the shift for the 2 boys. Magnitude of overshoot decreased with age and with increased use of the more advanced pattern. Immature balance and an initial need to visually guide the foot to the next step may be important factors in the timing of the pattern shift.