Women under the age of 35 years comprise only 5% of cases of breast cancer but because of the unexpected nature of the diagnosis this may be associated with delay. In the clinic, the diagnosis may not be immediate because of the low sensitivity of the components of triple diagnosis. The disease tends to be more aggressive with a higher incidence of poorly differentiated (grade III) carcinomas and more hormone insensitive (ER/PR negative) tumours. Additionally, the tumours are often rapidly proliferating and express abnormal p53. Women under 35 have an increased risk of local failure following breast cancer therapy, but this does not affect their cancer-specific survival. Young women treated with cyclophosphamide, methotrexate and fluorouracil chemotherapy have a greater risk of relapse and death than older premenopausal patients, especially if their tumours express oestrogen receptors, so endocrine treatment should also be offered under these circumstances.