This paper reports a prospective study which applied the Theory of Planned Behaviour (TPB) to the prediction of exercise behaviour over a six-month period. The study addressed a number of issues which have been identified in the literature on the TPB; these being the role of prior behaviour in the TPB, the distinction between desires and self-predictions, and the question of attitude variability. The findings showed prior behaviour to be the strongest predictor of exercise behaviour at six months. Contrary to expectations, the self-prediction measure was not found to be a better predictor of behaviour than the desire measure. Attitude variability was found to be related to perceptions of control. However, attitude variability was not found to moderate relationships between components of the TPB. The implications of the results for the development of the TPB are discussed.