PurposeAngiogenesis is crucial for tumor growth and is one of the hallmarks of cancer. In this study, we analyzed microvessel density, vessel median size, and perivascular a-SMA expression as prognostic biomarkers in breast cancer.MethodsDual IHC staining was performed where alpha-SMA antibodies were used together with antibodies against the endothelial cell marker CD34. Digital images of stainings were analyzed to extract quantitative data on vessel density, vessel size, and perivascular alpha-SMA status.ResultsThe analyses in the discovery cohort (n= 108) revealed a statistically significant relationship between large vessel size and shorter disease-specific survival (p= 0.007, log-rank test;p= 0.01, HR 3.1; 95% CI 1.3–7.4, Cox-regression analyses). Subset analyses indicated that the survival association of vessel size was strengthened in ER + breast cancer. To consolidate these findings, additional analyses were performed on a validation cohort (n= 267) where an association between large vessel size and reduced survival was also detected in ER + breast cancer (p= 0.016, log-rank test;p= 0.02; HR 2.3, 95% CI 1.1–4.7, Cox-regression analyses).ConclusionAlpha-SMA/CD34 dual-IHC staining revealed breast cancer heterogeneity regarding vessel size, vessel density, and perivascular a-SMA status. Large vessel size was linked to shorter survival in ER + breast cancer.Supplementary InformationThe online version contains supplementary material available at 10.1007/s10549-023-06974-4.