This study examined the responses of cerebellar patients and a group of age- and sex-matched control subjects to repeated changes in treadmill speed in order to test whether cerebellar patients can adapt their gait to this type of perturbation and, if so, whether their responses are comparable to those of controls. While the subject walked on the treadmill, a perturbation consisting of a sudden slowing of the treadmill followed by a sudden increase back to the original speed was applied repeatedly at a specific time during the step cycle. Both the control subjects and cerebellar patients were able to compensate for the perturbations by minimizing their postural sway and changing step length. However, the nature of the compensatory changes in step length differed between these subject groups. Control subjects compensated for the perturbation by consistently using the same leg to initiate the response to the perturbation and by adapting a pattern of stepping such that the EMG characterizing the response occurred in a manner that was entrained to the timing of the normal locomotor cycle. In contrast, the patients, although undergoing modifications in step length, employed a much less consistent motor pattern from trial to trial than that of the normal subjects. An inconsistent pattern among their responses was apparent in both the analysis of stepping and in the EMG activity of the gastrocnemius and anterior tibial muscles. These results suggest that, although the cerebellar patients can adapt their behavior in response to locomotor perturbations, they do not establish a motor pattern comparable to that employed by normal subjects.