Diabetes (DM) and hyperglycemia during chemotherapy increase the risk of toxicity, yet the prevalence and patterns of hyperglycemia in early-stage breast cancer (ESBC) patients undergoing chemotherapy with concurrent dexamethasone remain poorly understood. We conducted a prospective single-arm study using FreeStyle Libre Pro continuous glucose monitoring in patients with ESBC receiving chemotherapy from 12/2020-2/2022. Sensors measured interstitial glucose every 15 min and were reapplied every 2-3 weeks. Primary endpoints were (1) prevalence of hyperglycemia, and (2) for those with hyperglycemia, the proportion of time spent hyperglycemic. Secondary endpoints included baseline glucose tolerance by A1c, changes in glucose-related biomarkers, and changes in patient-reported neuropathy, quality of life, and fatigue. Analysis was stratified by baseline A1c. Among 20 evaluable patients, common chemotherapy regimens included docetaxel/cyclophosphamide, paclitaxel/trastuzumab, paclitaxel then doxorubicin/cyclophosphamide, docetaxel/carboplatin/trastuzumab/pertuzumab, and cyclophosphamide/methotrexate/fluorouracil. All patients received Dexamethasone. At baseline, 10 patients were euglycemic, 7 had pre-DM, and 3 had DM. All experienced hyperglycemia. All patients receiving chemotherapy for ESBC experienced hyperglycemia, with time spent hyperglycemic varying significantly by baseline A1c. Future research should explore approaches to and benefits of improving glycemic control during treatment in patients with baseline dysglycemia.