Background : A decrease in thermogenesis is suspected to be implicated in the energy expenditure reduction during breast cancer treatment. This study aimed to investigate the impact of chemotherapy on the metabolic activity of brown adipose tissue (BAT) and the link with weight variation. Methods : This was an ancillary analysis of a multicentre trial involving 109 HER2+ breast cancer patients treatedwith neoadjuvant chemotherapy. A centralised review of18F-FDG uptake intensity (SUVmax) in specific BAT regions(cervical and supraclavicular) was conducted on two PET-CT scans for each patient (before and after the first courseof chemotherapy). Results : Overall, after one course of chemotherapy a significant decrease of 4.4% in18F-FDG-uptake intensity wasobserved. It was not correlated to initial BMI, age or season. During chemotherapy, 10.1% (n= 11) of the patientslost weight (−7.7 kg ± 3.8 kg; ie,−9.4% ± 3.7%) and 29.4% (n= 32) gained weight (+ 5.1 kg ± 1.7 kg; ie, + 8.5% ±2.6%). Among these subgroups, only the patients who had gained weight underwent a significant decrease(13.42%) in18F-FDG uptake intensity (p= 0.042). Conclusion : This study is the first to highlight in a large cohort of patients the negative impact of chemotherapyon brown adipose tissue activity. Weight gain during chemotherapy could thus potentially be explained in part bya decrease in brown adipose tissue activity