Plexins, cell-surface receptors for semaphorins, are involved in cell adhesion and migration. In the previous work, we demonstrated that the loss of Plexin B1 expression is associated with poor outcome in breast cancer patients. The goal of the present study was a validation of Plexin B1 expression in a large scale microarray dataset from n=1086 breast cancer patients. Plexin B1 correlates with ER status (p<0.001) and is of prognostic significance only in ER positive (p=0.024) but not in ER negative samples (p=0.85). Among ER positive tumours, the loss of Plexin B1 expression is associated with a positive ErbB2 status (p=0.05) and a high Ki67 expression (p=0.016) in univariate analysis. Multivariate Cox regression including all standard parameters among ER positive tumours revealed that Plexin B1 (HR 1.59, 95% confidence interval (CI) 1.03-2.47, p=0.036) remains a significant prognostic marker besides tumour size (HR 2.27, 95% CI 1.33-3.89, p=0.0028) and Ki67 (HR 1.78, 95% CI 1.12-2.84, p=0.0149). Interestingly, the prognostic value of Plexin B1 was pronounced in low proliferating ER positive tumours otherwise characterised by a low risk of recurrence. In conclusion, this study confirms our previous observations suggesting Plexin B1 as a new prognostic marker in ER positive breast cancers.