Patients with degenerative cerebellar disease were compared to healthy controls in their ability to adapt behaviour to temporal contingencies, both according to instructions and according to acquired experience. Participants had to press the cued key whenever the inside of a clock face changed its colour, which could occur when the pointer, rotating once every 4s, was at "10h" or at "12h" or at "2h". Probabilities varied between blocks at which of these three time points the colour change occurred, with participants being instructed accordingly. Response times correlated intraindividually with these instructed "a priori" probabilities in control participants only. Subjectively, at any moment, probabilities of occurrence depend on whether the imperative colour change had occurred before, thus may be better described by conditional ("a posteriori") probabilities. Indeed, when response times were correlated to a posteriori rather than a priori probabilities, correlations increased in both groups equally from their different a priori levels. The amplitudes of preparatory EEG negativity before responding tended to obey to the same relationships, suggesting that the difference between groups was not due to pure motor impairment. Thus, these data suggest that patients with cerebellar atrophy are more impaired in implementing and using task-relevant information in a top-down manner than in learning to modify task-relevant contingencies.