Variability of gait may be regarded as a sign of adaptability and thus a requirement for successful locomotion, or as a sign of impaired balance control. In this study we examined the role of step width variability (SWV) and interstride trunk acceleration variability in two groups of fit and frail old people. We investigated the association of these measures and how they differentiated the two groups. We examined 33 fit older adults (mean age 73 years, S.D. 3.3 years) and 32 frail old people (mean age 80 years, S.D. 4.0 years). Subjects performed timed walking at different speeds ranging from very slow to very fast. SWV was measured from footprints. Trunk accelerations were registered by a triaxial accelerometer and interstride trunk acceleration variability assessed by an unbiased autocorrelation procedure. All measures were normalized to a walking speed of 0.9 m/s to avoid the confounding effect of gait speed on speed dependent gait parameters. SWV demonstrated low association with the trunk variability measures, and did not differ between groups. The frail group had lower mediolateral (P=0.015), but higher vertical (P=0.015) and anteroposterior (P<0.02) trunk variability than the fit group. Trunk variability classified 80% of the subjects correctly into their respective group (sensitivity=0.75, specificity=0.85). The findings are compatible with a notion that mediolateral interstride trunk variability represents a different aspect of motor control than variability in the direction of propulsion.