This study compared individual step and averaged spatial and temporal gait parameters measured with an instrumented walkway system (GAITRite) with a three-dimensional motional analysis system (Vicon-512). Ten subjects aged 54-83 years (mean 66.5 years) who had undergone knee replacement surgery participated. Subjects walked across the GAITRite walkway at self-selected comfortable and fast speeds at the same time as the Vicon system recorded the motion of reflective markers attached to the subjects' shoes. Walking speed, cadence, step length and step time variables, averaged across one walk for both systems, showed an excellent level of agreement with intra-class correlation coefficients (ICCs) between 0.92 and 0.99 and repeatability coefficients (RCs) between 1.0% and 5.9% of mean values. Step length and step time variables recorded for each footstep also showed good agreement between the systems at both comfortable and fast speeds (ICCs between 0.91 and 0.99; RCs between 2.6% and 7.8%). Frequency distributions showed that individual step values were within 1.5 cm and 0.02 s on the majority (80-94%) of occasions. These data indicate that the GAITRite system is a valid tool for measuring both averaged and individual step parameters of gait. It is also valid for use in older subjects following knee joint replacement surgery.