Thirteen patients with bilateral cerebellar disease and 12 patients with unilateral cerebellar disease were instructed to execute movement sequences in response to a simple reaction signal. Each to-be-executed sequence consisted either of a single, two, or three keypress components. Evidence for cerebellar involvement in the execution of programmed responses was sought in the pattern of response onset times and interkeypress times. Patients with mild bilateral cerebellar dysfunction or mild unilateral dysfunction, and neurologically unimpaired subjects showed increases in response onset time as sequence length increased from L = 1 to L = 3. In contrast to this, there were negligible or no effects of sequence length on response onset time in patients with moderate bilateral cerebellar dysfunction and in patients with moderate unilateral cerebellar dysfunction who responded with the hand ipsilateral to the lesion. Furthermore, cerebellar dysfunction was associated with significantly slower interkeypress reaction times. These results support the hypothesis that the translation of a programmed sequence of responses into action involves cerebellar structures which schedule a sequence of ordered responses before onset of movement.