Understanding exercise self-efficacy in breast cancer patients during treatment is important for enhancing physical activity adherence. Therefore, the primary study purpose was to determine, among breast cancer patients during treatment, the psychometric properties of scales to measure exercise barrier and task self-efficacy. The study also aimed to determine the following: (1) level of self-efficacy, (2) associations between barrier and task self-efficacy, and (3) associations between self-efficacy and patient age, race, and treatment type. Eighty-six female breast cancer patients recruited from a medical oncologist's office completed the scales once, and 46 repeated the scales 2 weeks later. The majority were Caucasian (95%), with 26% receiving chemotherapy, 64% hormonal therapy alone, and 5% radiation/other. The mean age was 59±14 years. The Cronbach's alpha for the nine-item barrier self-efficacy scale was 0.96, with a test-retest correlation of 0.89 (p<0.001). The Cronbach's alpha for the four-item task self-efficacy scale was 0.89, with a test-retest correlation of 0.83 (p<0.001). The mean barrier self-efficacy was slightly to moderately confident, with the lowest confidence reported in the ability to exercise when nauseated. The mean task self-efficacy was slightly to moderately confident, with the lowest confidence reported in the ability to jog for 10 min without stopping. Although no significant associations were found between self-efficacy and participant's race or treatment type, lower task self-efficacy was associated with older age (r=-0.36, p=0.001). Both self-efficacy scales demonstrated good internal consistency and test-retest reliability. Self-efficacy may be a useful target for physical activity interventions among breast cancer patients during treatment. © Springer-Verlag 2005.