Oncologists recommend chemotherapy to postmenopausal women with adverse prognostic factors, but predictors of the benefit of chemotherapy are mainly based on mortality from symptomatic cancer trials. From 1990 to 1998, 1475 breast cancers (875 screen detected cancers [SDBCs]: 600 symptomatic) were treated in women aged 50-65 years and prognostic factors compared with cancer mortality. Median follow-up was 110 months. The Nottingham Prognostic Index (NPI) was calculated for 6737 breast cancers which were part of the Association of Breast Surgery (ABS) 2001/2002 Audit of SDBCs to validate survival figures. Ten year survival was 92.1% for SDBC and 77.6% for symptomatic cancers. Adjusting for baseline factors, SDBCs had a reduced mortality (RR = 0.42 (0.31-0.57), independent of grade, node status and tumour size. Oestrogen receptor (ER) positive SDBC had a lower annual mortality rate (0.6%) compared with symptomatic (4.3%: P < 0.001) or ER negative SDBC (1.8%). Epithelial proliferation was lower in SDBC in all NPI groups compared with symptomatic cancers (P ≤ 0.001). Grade, node status, ER status, size and mode of detection predicted survival. Survival for each NPI group was better for SDBC. For ER positive SDBC in the Moderate Prognostic Group 1 (MPG1), 10 year mortality was 6.4% compared with 17.6% in symptomatic (P = 0.001). NPI on 6,737 operable SDBC confirmed similar mortality in all groups (4% mortality in MPG1 group). SDBC have lower mortality than symptomatic due to a lower proliferative index. The use of adjuvant chemotherapy is over-treatment for ER positive SDBCs with Good Prognostic Group (GPG) and MPG1 NPI scores.