A prognostic index, previously derived in a group of 387 patients with primary breast cancer, has been recalculated for the same patients with over 5 years further follow-up and shown to be unchanged. The prognostic index has also been applied prospectively to a further group of 320 patients and shown to be similarly effective in identifying patients with either a very good or a very poor prognosis. It has been verified that the index applies to patients with primary breast cancer. Patients have now been divided into 5 prognostic groups, predicting 11% of patients with an almost normal survival and a further 10% with a very poor prognosis. The index is used to stratify patients to study the effects of treatment regimes within groups of similar patients.