Preliminary evidence indicates that physical exercise may be an effective strategy for helping cancer patients cope with the negative side-effects of their treatment. The purpose of the present study was to examine the utility of the theory of planned behavior in understanding cancer patients' motivation to exercise during treatment. A total of 164 women who had been diagnosed with breast cancer within the previous 2 years participated in the study. A retrospective design was used wherein these women were asked to recall their beliefs and exercise behavior during cancer treatment using a self-administered, mailed questionnaire. Results showed that: (1) the salient beliefs of breast cancer patients concerning exercise were different from those of the healthy population; (2) intention and perceived behavioral control were significant determinants of exercise during cancer treatment; and, (3) attitude and subjective norm were significant determinants of intention. It was concluded that the theory of planned behavior may be a viable framework on which to base interventions designed to promote exercise during cancer treatment. The results of this study are preliminary, however, and more sophisticated research designs to examine this question are warranted.