It has been found that the chemical changes that take place in a patient's body during the development of inflammation may provide an environment which stimulates cancer cells. One step in the development of inflammation is the production of certain chemical substances which are important in the formation and spread of tumours. These are called prostaglandins. Cyclo-oxygenase II (COX-2) is an enzyme (a substance that speeds up chemical changes in the body) involved in the production of these prostaglandins and although it is not usually present in most tissues it is made at the sites of inflammation. Celecoxib is a selective Non-Steroidal Anti Inflammatory Drug (NSAID) which works by blocking the action of the COX-2 enzyme, leading to a decrease in the production of prostaglandins and a reduction in inflammation. The purpose of this study is therefore to find out if celecoxib can be used after breast cancer treatment (chemotherapy and/ or radiotherapy) to reduce inflammation and thus reduce the ability of new tumours to grow and survive. 2590 women with primary breast cancer will be recruited in this study from several locations in the United Kingdom and Germany. Eligible patients will be randomly allocated a treatment group, which can be celecoxib or placebo. Both treatments are taken orally (celecoxib 400mg daily, placebo 2 tablets daily) for a total of 2 years. In addition, hormone receptor positive patients will receive endocrine treatment as per local practice. Patients will prematurely discontinue treatment with celecoxib/placebo if disease progression is confirmed or if patients experience unacceptable toxicity. Patients will be seen every 6 months for the first 3 years and then off treatment follow-up is carried out annually. Participating patients will also be given the option to take part in the pathology sub-study by donating a sample of the tumour tissue collected at the time of the primary surgery.