Relatively few studies have examined how uneven surfaces affect human gait. To study this, along with the effects of advancing age and low light conditions, we measured step width and step time variability, as well as comfortable gait speed, in 12 healthy young women (YW) and 12 healthy older women (OW) as they walked at a comfortable speed along a 10-m walkway. Ten trials were completed for each cell of a 2 x 2 factorial design: (1) flat surface with regular lighting; (2) flat surface with low lighting; (3) irregular surface with regular lighting; and (4) irregular surface with low lighting. Effects of surface type, incident lighting, and age on gait parameters were tested via repeated measures ANOVA. Surface type significantly affected step width variability (P < 0.001) and step time variability (P < 0.001). Light level showed no significant effect on any of the gait parameters. One significant age group difference was found: step width variability was significantly greater in OW than YW (P = 0.010). We conclude that the irregular surface had a greater effect on the gait variability of healthy OW than on that of healthy YW.