With this research, we sought to establish the accuracy of stride velocity data collected by the s3 Stride Sensor. Participants walked along a GAITRite mat at self-selected slow, preferred, and fast velocities, with two s3 Stride Sensors attached to their right foot. The start position was systematically varied such that the GAITRite system would record the second through sixth strides at each walking velocity. Both slow and preferred walking velocities were underestimated by 14% relative to the GAITRite (p less than 0.05), while independent of walking velocity, Strides 2 and 3 were underestimated by 26% and 9% (p less than 0.05), respectively. Researchers should use caution when interpreting data collected at slow and preferred walking velocities and during the first three strides. (Contains 1 figure and 3 tables.)