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The purpose of these experiments was to examine the role of the human cerebellum in the acquisition and retention of conditioned reflexes. Normal human subjects and patients with cerebellar lesions were tested for their capacity to acquire, retain and express conditioned eyeblink responses. In acquisition tests, subjects were trained in a delay classical conditioning paradigm using a tone conditioned stimulus and a midline forehead tap as an unconditioned stimulus. While normal subjects developed anticipatory eyeblinks to the tone in one session, patients with cerebellar lesions failed to acquire conditioned responses in four consecutive training sessions. The conditioning deficit was bilateral even in patients with a unilateral cerebellar pathology. The same groups of subjects were tested for the presence of eyeblinks to a visual threat. In these experiments, both normal subjects and patients with cerebellar lesions exhibited a high level of responding when they saw an object approaching their face. These eyeblinks to the visual threat are probably naturally acquired conditioned responses because they extinguish in normal subjects if they are not reinforced by the unconditioned cutaneous stimulus. In addition, the stimulus of seeing an approaching object blocks the acquisition of classically conditioned eyeblinks to a new conditioned stimulus in normal subjects. These data imply that patients with cerebellar lesions who cannot acquire new classically conditioned responses are able to retain and express conditioned eyeblinks which were acquired before the onset of the pathology. Consequently, cerebellum-dependent neural substrates which are involved in learning new conditioned reflexes do not seem to be required for the storage of naturally learned conditioned responses.