Behavioural theorists have identified attitudes, perceived norms and self-efficacy as the important determinants of people's intentions to engage in a given behaviour. Because intentions predict behaviour, these same variables also account for a considerable amount of the variation in behaviour. Nevertheless, there is often a substantial proportion of the population who do not act on their intentions. While a recently proposed integrative theory of behaviour suggests that these ‘failures’ are due either to a lack of skills and/or to the presence of environmental constraints, it has also been argued that the determinants of intention may have a direct, as well as in indirect, effect on behaviour. This paper uses data from a longitudinal study (Project RESPECT) to explore the extent to which attitudes, perceived norms and self-efficacy explain why some people do and others do not act on their intentions to engage in a health protective behaviour. Although the data provide further evidence that these three variables account for a significant proportion of the variance in intentions (and behaviour), they perform poorly when predicting behaviour for persons with pre-existing high intentions. It may be reasonable to ask whether a ‘new’ theory is needed to explain why some people do, and some people do not, act on their intentions.