Background: Obesity is associated with breast cancer development and worse survival. Obesity can initiate, promote, and maintain systemic inflammation via metabolic reprogramming of macrophages that encircle adipocytes, termed crown-like structures (CLS). In breast cancer patients, CLS are present in 36-50% of patients and have been associated with anthropometric parameters. Here we focus on HER2+ breast cancer. The role of adiposity in HER2+ breast cancer is conflicting which may be attributed to the tumour heterogeneity. Adiposity has also been shown to affect the local immune environment of solid tumours. However, the prognostic significance of CLS in HER2+ breast cancer is still unknown. Methods: We investigated the prognostic significance of CLS in a cohort of 219 patients with primary HER2+ breast cancer who were diagnosed between 1982 to 2012 in Southampton General Hospital. This cohort includes 76 HER2+ trastuzumab naïve patients and 143 HER2+ patients treated with adjuvant trastuzumab. We stained FFPE tumour samples for the expression of CD68, CD16 and CD32B on CLS and correlated these to clinical outcomes. CLS were defined as CLS within distant adipose tissue, CLS within the adipose-tumour border (B-CLS) and intratumoural CLS. CLS were quantified manually in full face sections by two independent scorers and descriptive and Cox regression analysis was carried out. Results: A total of 201 tumours were suitable for CLS analyses. The median follow-up was 34.74 months (range, 0.43-299.08). In the trastuzumab naive cohort, B-CLS≤1 and B-CLS > 1 were present in 37 (52.11%) and 34 (47.89%), respectively. In the trastuzumab treated cohort, B-CLS≤1 were identified in 69 (53.08%) and B-CLS > 1 were found in 61 (46.92%) of the tumours. CLS were more commonly found in the adipose-tumour border (60.89%) rather than in the distant adipose tissue (36.14%) or intratumorally (14.36%). The presence of any CLS was significantly associated with BMI≥25 kg/m² (p = 0.018). There was strong evidence of association between CD68+CD32B+ B-CLS and BMI≥25 kg/m² (p = 0.007). Co-expression of CD16 and CD32B by B-CLS was more frequent in patients with BMI≥25 kg/m² (p = 0.036). Survival analysis showed shorter time to metastatic disease in patients with CD68+ B-CLS > 1 (p = 0.011) in the trastuzumab treated cohort. Subgroup analysis revealed that in the BMI≥25 kg/m² group, patients with CD68+ B-CLS > 1 had shorter time to metastatic disease compared to patients with B-CLS≤1 (p = 0.004). Multivariate cox regression showed that B-CLS > 1 is an independent prognostic factor for shorter time to metastatic disease in patients with primary HER2+ breast cancer that received adjuvant trastuzumab (HR 6.81, 95%CI (1.38-33.54), p = 0.018). Conclusions: B-CLS can be potentially used as a predictive biomarker to optimize the stratification and personalisation of treatment in HER2-overexpressed breast cancer patients.