Aquaporin1 (AQP1) is a water channel protein which facilitates water flux across cell membranes. AQP1 is found in epithelial and endothelial cells in various tissues. There is increasing evidence that AQP1 is expressed in malignant tumours and that it may play a role in tumour angiogenesis, cell migration and metastasis. We studied the immunohistochemical expression of AQP1 in a cohort of 203 invasive breast carcinomas with long-term follow up. AQP1 expression was detected in 11 cases (5.4%), and showed a significant correlation with high tumour grade, medullary-like histology, "triple-negativity", as well a cytokeratin 14 and actin expression. In univariate analysis, AQP1 was associated with a significantly poorer prognosis. Multivariate analysis showed that AQP1 expression has an independent predictive value for outcome if stratified by age, tumour size, lymph node status, histological grade and ER status. AQP1 expression in invasive breast carcinomas is associated with a basal-like phenotype and poor prognosis.