ONCOPOOL is a retrospectively compiled database of primary operable invasive breast cancers treated in the 1990s in 10 European breast cancer Units. Sixteen thousand and nine hundred and forty four cases were entered, with tumours less than 5 cm diameter in women aged 70 or less (mean age 55). DATA: Data were date of birth, mode of diagnosis, pathology (size, lymph node status, grade, type, lympho-vascular invasion and hormone receptor) and therapies and outcome measures: first local, regional or distant recurrences, contralateral primary, date and cause of death. TUMOUR CHARACTERISTICS: Mean diameter 1.8 cm, 66% lymph node negative, 24% 1-3 lymph nodes involved and 10% had 4 or more involved. Grade 1, 29%; Grade 2, 41%; and Grade 3, 30%. Polynomial relationships were established between grade, stage and size. Seventy-five percent were oestrogen receptor (ER) positive. ER closely related to grade. OUTCOMES: Overall Survival was 89% at 5 years from diagnosis, 80% 10 years and 73% 15 years; Breast Cancer-Specific survivals were 91%, 84% and 79%. Survival strongly related to the Nottingham Prognostic Index (NPI). Cases detected at screening had 84% 10-year survival, those presenting symptomatically 76%. ER positive cases treated with adjuvant hormone therapy had a reduction in risk of death of 13% over those not receiving adjuvant therapy (p=0.000). ER negative cases treated with chemotherapy showed a risk reduction of 23% over those not receiving chemotherapy (p=0.000).