Gait variability has been correlated with fall risk in the elderly. Older adults typically display greater variability than young adults, but the cause of this increase is unclear. Slower walking leads to greater variability in young adults, but slow speeds are also typical in older adults. Increased variability in older adults may result from slower walking speeds, or possibly from other factors related to aging. We tested whether greater variability in healthy older adults could be attributed directly to slower walking speed. Eighteen healthy older adults (age 72+/-6) and 17 gender-, height- and weight-matched young adults (age 23+/-3) walked on a treadmill at speeds of 80-120% of their preferred speed. Variability of spatio-temporal gait measures, lower extremity joint angles, and trunk motions were quantified, along with bilateral isometric leg strengths and passive joint ranges of motion. Preferred walking speeds were not different between our healthy elderly and young subjects. In both groups, variability was speed-dependent for stride time, frontal hip and knee motions, knee internal/external rotations, and all trunk motions (p<0.002). Older adults exhibited greater variability (p=0.0003) for trunk roll, independent of changes in speed. Step length (p=0.005), stride time (p=0.018), and trunk pitch (p=0.022) exhibited similar trends. This greater variability was explained by decreased leg strength and passive ranges of motion. Thus, the greater variability observed in the gait of older adults may result more from loss of strength and flexibility than from their slower speeds.