Purpose/Objectives: To measure the association between physical activity and social cognitive theory constructs during breast cancer treatment. Design: Cross-sectional survey. Setting: Midwestern, academic oncology clinic. Sample: 21 primarily Caucasian (90%) female patients with breast cancer undergoing treatment. 76% were ≥ 50 years old; 76% had stage I or II disease. 17 completed the study. Methods: Survey (structured interview or self-administration), chart audit, pedometer, and seven-day physical activity recall. Main Research Variables: Steps per day, energy expenditure, self-efficacy, barriers, partners and role models, prior physical activity counseling, physical activity knowledge, pretreatment physical activity, outcome expectations and values, goals, reinforcement management, and emotional well-being. Findings: A higher average of steps per day was significantly associated with having an exercise role model and higher annual income. A higher daily energy expenditure (kilocalories per kilogram body weight per day) was significantly associated with higher barrier self-efficacy, higher task self-efficacy, having an exercise partner, having an exercise role model, higher physical activity enjoyment, and lower negative value score. Conclusions: Social cognitive theory may provide a useful framework for understanding physical activity among patients with breast cancer during treatment, but correlation strength varies with physical activity measurement type. Implications for Nursing: Social cognitive theory and physical activity during breast cancer treatment warrant additional study with larger sample sizes and multivariate analyses. Interventions to increase physical activity among patients with breast cancer may use social cognitive theory and assess theory constructs as potential mediators or moderators in intervention evaluation.