We have used immunohistochemical staining to assess the expression of cyclin D1 in formalin-fixed sections of 345 breast carcinomas, dating back 20 years. Clinical follow-up data were available on all patients. Approximately 50% of the tumours showed excessive nuclear staining for cyclin D1 as compared with normal epithelium. Some tumours showed strong cytoplasmic staining in the absence of nuclear staining, and around 25% of the tumours were judged to be negative for nuclear cyclin D1. Contrary to expectations, moderate/strong staining for cyclin D1 was associated with improved relapse-free and overall survival relative to patients whose tumours stained weakly or negatively. Conversely, tumours that were considered negative for cyclin D1 staining had an adverse prognosis, and the poor outcome was further accentuated if the tumours were also oestrogen receptor-negative. A possible explanation for our findings is that tumours in which cyclin D1 levels are abnormally low may have sustained mutations in other genes, such as RBI and that it is this abnormality that has the more significant impact on survival from breast cancer.