The purpose of this study was to compare vertical ground reaction forces walking overground with vertical foot-belt forces for treadmill gait. Twenty-four subjects walked overground and on a treadmill at three speeds (slow, normal, and fast), and at comparable cadences and stride length at each of the speeds. Treadmill and overground vertical force curves were normalized to 100% of stance time and compared using Person's product moment correlation. Selected measures from vertical force records were compared between the two modes of locomotion via repeated measures ANOVA (P < 0.05). Post-hoc analysis consisted of paired t-tests with Bonferroni correction. All comparisons were made across conditions (treadmill vs overground) at each of the three walking speeds. The pattern of reaction forces were similar. Correlation between curves were 0.998, 0.983, and 0.983 for the slow (1.03-1.05 m.s-1), normal (1.40-1.44 m.s-1) and fast (1.65-1.71 m.s-1) walking trials. Small (5-9%) but significant differences in force magnitude for the two forms of locomotion were evident during mid-stance for normal (P = 0.00009) and fast (P = 0.0007) walking speeds and in late stance for normal (P = 0.0014) and fast (P = 0.0005) trials. Although the patterns of the vertical reaction forces for the two forms of locomotion were nearly identical, small but significant differences in selected force magnitudes were evident. The interpretation of locomotion data collected on a treadmill should consider that forces during mid- and late-stance may be different than if the subject walked overground.