Cyclooxygenase type-2 (COX-2) is overexpressed in malignant tumours including breast cancers, though the mechanism of upregulation is unclear. This study aimed to determine COX-2 expression in ductal carcinoma in situ (DCIS) in comparison to invasive breast cancer (IBC) and normal breast, and also to investigate the relationship of COX-2 expression with HER-2 expression, oestrogen receptor (ER), tumour grade and cellular proliferation (Ki67) in DCIS. Cyclooxygenase type-2, HER-2, ER and Ki67 expression were determined by immunohistochemistry on paraffin tissue sections of DCIS (n=187), IBC (n=65) and normal breast reduction tissue (n=60). Cyclooxygenase type-2 expression in DCIS (67%, P<0.001) and IBC (63%, P<0.001) was significantly greater than in normal breast (23%). There was no difference in COX-2 expression level between DCIS and IBC (P=0.87) or between normal breast from reduction mammoplasty tissue and normal breast ducts around DCIS (22%, P=0.29). In DCIS, COX-2 expression was associated with higher cellular proliferation rates (P<0.0001), nuclear grade (P=0.003), with ER negativity (P=0.003) and with HER-2 positivity (P<0.0001). Cyclooxygenase type-2 expression is upregulated in in situ breast cancer and is associated with surrogate markers of an aggressive DCIS phenotype including nonoestrogen-regulated signalling pathways. Cyclooxygenase type-2 inhibition may potentially prevent the development of ER-positive and ER-negative breast cancers.