Circulating tumour DNA (ctDNA)-based detection of molecular residual disease (MRD) presents a strategy to identify patients at high risk of relapse. In this article, we profile early breast cancer patients with an ultrasensitive, whole genome sequencing (WGS)-based, tumour-informed ctDNA platform.We analysed 617 plasma samples (median 8, range 2-14) from 78 patients (23 triple-negative breast cancer, 35 human epidermal growth factor receptor 2-positive, 18 hormone receptor-positive, and 2 unknown). Samples were collected at diagnosis before therapy, cycle 2 of neoadjuvant chemotherapy, post-surgery after neoad'juvant therapy if administered, every 3 months during the first year, and every 6 months thereafter. Plasma DNA was analysed using the NeXT Personal MRD platform, a tumour-informed WGS approach to produce personalized ctDNA sequencing panels tracking a median of 1451 variants per patient. MRD detection was correlated with clinical outcomes.ctDNA was detected at levels ranging from 2.19 parts per million (PPM) to 204 900 PPM (median 405 PPM), with 39% of all ctDNA detections in the ultra-low range