Representative tumour sections from 468 patients with invasive breast cancer were immunostained for cyclooxygenase-2 (COX-2) and evaluated. The relationships between COX-2 expression, clinical outcome and various clinicopathological variables, including tumour vascularity and disseminated tumour cells (DTC) in the bone marrow were examined. COX-2 expression in invasive breast carcinoma cells was positively associated with oestrogen receptor and/or progesterone receptor positivity (p<0.001). Triple-negative tumours showed no/low COX-2 expression more frequently than other tumour types (p<0.001). Expression of COX-2 was not associated with breast cancer-specific survival (p=0.49, log-rank) or distant disease-free survival (p=0.67, log-rank) for all patients, including lymph node-negative, untreated patients (p>0.14, log-rank). There was also no significant association between COX-2 expression and histological grade, tumour size, nodal status, DTC in bone marrow, p53, HER2, or tumour vascularity. In conclusion, COX-2 expression in this series was associated with the presence of hormone receptors. Low COX-2 expression was observed in triple-negative breast carcinomas.