Estrogen receptor-positive (ER+) breast tumours are often treated with anti- estrogen (AE) therapies but frequently develop resistance. Cancer Stem Cells (CSCs) with high aldehyde dehydrogenase (ALDH) activity (ALDH+ cells) are reported to be enriched following AE treatment. Here we perform in vitro and in vivo functional CSC assays and gene expression analysis to characterise the ALDH+ population in AE resistant metastatic patient samples and an ER+ cell line. We show that the IL1 signalling pathway is activated in ALDH+ cells and data from single cells reveals that AE treatment selects for IL1R1-expressing ALDH+ cells. Importantly, CSC activity is inhibited by Anakinra, a synthetic IL1R1 antagonist, in AE-resistant models. Moreover, we demonstrate that increased expression of IL1R1 is observed in the tumours of patients treated with AE therapy and predicts for treatment failure. Single-cell gene expression analysis revealed that at least 2 sub-populations exist within the ALDH+ population, one proliferative and one quiescent. Following AE therapy, the quiescent ALDH+IL1R1+ population is expanded, which suggests CSC dormancy as an adaptive strategy that facilitates treatment resistance. Supporting this, analysis of AE resistant dormant tumours reveals significantly increased expression of ALDH1A1, ALDH1A3 and IL1R1 genes. Thus, our work establishes that targeting of ALDH+IL1R1+ cells may reverse AE resistance in patients with minimal residual disease.