Timing variability on a repetitive tapping task was studied in subjects with unilateral cerebellar lesions. During unimanual tapping, within-hand variability was larger when tapping with the ipsilesional hand in comparison to tapping with the contralesional hand. However, variability in the impaired hand was greatly reduced when subjects tapped with two hands together. The improvement in within-hand variability during bimanual tapping was associated with a reduction in central variability rather than response implementation variability according to the two-process model of Wing and Kristofferson (1973). It is proposed that (1) each half of the cerebellum independently regulates the temporal aspects of movements on the ipsilateral side and (2) temporal coupling constraints require these separate signals to be integrated prior to response implementation for bimanual movements.