AIMS: To produce a simplified prognostic index for breast cancer. METHODS: A retrospective study of 105 cases of primary infiltrating breast adenocarcinoma (not otherwise specified) was performed. Mitotic counts and semiquantitative assessment of tumour necrosis, fibrosis tubule formation, nuclear pleomorphism were made in histological sections of the primary tumour. Statistical analysis was performed to determine which of these parameters best predicts the observed survival. RESULTS: Mitotic count and necrosis correlated best with survival. This allowed the formation of a simple prognostic index based on these two parameters. CONCLUSIONS: This new prognostic index, with four tumour grades, seemed to be superior to Bloom's grading method, with greater separation of the prognostic groups. In particular, there is clear segregation of a group of patients with a distinctly poor prognosis.