A prospective study applying the theory of planned behavior (TPB) to the prediction of attendance at health checks is reported. Conducted in a single general practice, 307 patients completed questionnaires based on the TPB, and were invited to attend a health check. The role of prior behavior in the TPB was assessed in 2 ways. First, it was used to assess the sufficiency of the TPB. The addition of prior attendance behavior led to a significant increase in the prediction of current behavior. Second, its role as a moderator variable was assessed. While the TPB was unsuccessful in predicting attendance behavior among prior attenders, it was able to do so among prior nonattenders. The results are discussed in relation to recent work on decision-making processes.