Background: CANTO (CANcer TOxicities - NCT01993498) is a French multicenter prospective longitudinal study dedicated to the quantification and characterization of side effects after treatment for patients with stage I-III breast cancer, and to the development of predictors of toxicities. Methods: The CANTO study has included > 12,000 patients, assessed at diagnosis, and 3-6 months (T0), 15 months (T12), and year 3 and 5 after treatment completion. In the current report, we focus on key toxicities at T0 and T12 according to chemotherapy (CT) administration. For each side effect, 4 populations were defined according to its occurrence at T0 and/or T12 (no/no, no/yes, yes/no, yes/yes). Results: We analyzed 4684 patients with T0 and T12 consolidated data. Median age at diagnosis was 57y (22-89). Patients (pts) had HR+/HER2-, HER2+ or triple negative tumors in 78.9%, 12.4% and 8.7% of cases, respectively. Overall, 2516 pts (53.7%) received CT. Most CT pts (81%) received a sequential anthracyclines– taxanes schedule. As an example, a high proportion of patients presented neurological symptoms including cognitive symptoms, sensory or motor neuropathy, paresthesia, headache, etc (all grades) at either T0 or T12. Overall, CT was strongly associated with neurotoxicity at all times (OR = 2.27, p < 0.0001). However, proportions of patients with neurological side effects changed between T0 and T12. The table shows the proportions of the 4 categories of pts in the CT and no CT groups. Furthermore, at T12, neurological symptoms remained more frequent in the CT group, whether pts had symptoms at T0 (CT vs no CT, 81% vs 77%, p = 0.007) or not (CT vs no CT, 41% vs 36%, p = 0.03). Similar temporal trends were observed (with specific percentages for each considered side effect) for detailed neurological toxicities, pain and joint/bone toxicity (stratified on endocrine therapy), gastro-intestinal, pulmonary and cardiac toxicities. Conclusions: Overall, symptoms burden is extremely high at T0 and T12 after treatment, and much higher in pts receiving CT. A high temporal variability was observed in all subsets, including a clinically meaningful delayed onset of e.g. neurological side effects. Clinical trial information: NCT01993498