The movement of the center of mass (COM) during human walking has been hypothesized to follow a sinusoidal pattern in the vertical and mediolateral directions. The vertical COM displacement has been shown to increase with velocity, but little is known about the mediolateral movement of the COM. In our evaluation of the mediolateral COM displacement at several walking speeds, 10 normal subjects walked at their self-selected speed and then at 0.7, 1.0, 1.2, and 1.6 m/s in random order. We calculated COM location from a 15-segment, full-body kinematic model using segmental analysis. Mediolateral COM displacement was 6.99 +/- 1.34 cm at the slowest walking speed and decreased to 3.85 +/- 1.41 cm at the fastest speed (p < 0.05). Vertical COM excursion increased from 2.74 +/- 0.52 at the slowest speed to 4.83 +/- 0.92 at the fastest speed (p < 0.05). The data suggest that the relationship between the vertical and mediolateral COM excursions changes substantially with walking speed. Clinicians who use observational gait analysis to assess walking problems should be aware that even normal individuals show significant mediolateral COM displacement at slow speeds. Excessive vertical COM displacement that is obvious at moderate walking speeds may be masked at slow walking speeds.