This study investigated the effect of virtual reality and manipulation of the speed of the virtual environment while walking on a treadmill, both in people post-stroke and healthy people. Sixteen people post-stroke and 16 healthy controls walked on a self-paced treadmill. After 2 habituation trials (without and with virtual reality), participants walked 3 more trials under the following conditions of optic flow: (i) walking with a matching virtual environment, (ii) walking with a slower virtual environment, and (iii) walking with a faster virtual environment. The addition of virtual reality did not influence the gait pattern of people post-stroke or healthy people. When the speed of the virtual environment was manipulated, people post-stroke altered their gait pattern by changing their walking speed. People increased their walking speed with a slower virtual environment, and decreased their speed with a faster virtual environment. Incorporating such manipulations for treadmill training could be feasible, but further research is needed.