OBJECTIVE To describe changes in physical activity levels related to health-enhancing physical activity guidelines during and after adjuvant chemotherapy in a cohort of Swedish patients with breast cancer. METHODS Exploratory clinical prospective longitudinal study with 6 and 12 months of follow-up. METHODS Forty women treated for breast cancer with adjuvant chemotherapy were included, and 33 women completed the study. METHODS The primary outcome was the proportion of participants meeting the health-enhancing physical activity guidelines, both in relative terms (perceived exertion using Borg's rate of perceived exertion scale) and absolute terms (metabolic equivalent). RESULTS The proportion of participants meeting the guidelines when leisure-time physical activity was measured in terms of absolute intensity was 81 (25/31) to 94% (31/33) during chemotherapy, 93% (29/31) at 6-month follow-up and 87% (26/30) at 12-month follow-up. Women who had been physically active before their cancer diagnosis and women who had received information about physical activity were more physically active during chemotherapy. CONCLUSIONS Swedish patients with breast cancer are generally more physically active during adjuvant chemotherapy than has been reported previously in international studies. Physical activity habits prior to cancer diagnosis and information about the benefits of physical activity appear to be important factors for higher levels of physical activity during and after chemotherapy.