Various histological changes in the breast have been associated with an increased risk of development of breast cancer; these changes have been identified in non-involved tissue in cancer-containing breasts, suggesting that factors promoting the development of carcinoma may have a field effect. Previous work has identified alterations in growth factor receptors and integrins in cancer-containing breast tissue. In the present study, proliferation and apoptosis are examined. Non-involved breast tissue from 104 women taken at least 4 cm away from a carcinoma and normal/benign tissue from 105 age-matched women were studied. Proliferation was assessed using MIB-1 immunohistochemistry and labelling for histone mRNA, as a marker of S-phase. In situ end-labelling was used to identify apoptosis; any non-labelled apoptotic bodies were also counted. No differences were found between the non-involved tissue and the control group for MIB-1 index and histone index. The apoptotic index was higher in the control group than in the cancer-containing breasts, being greater for ducts than for acini. When the apoptotic index/MIB-1 index and apoptotic index/histone index were considered, the mean for both was lower in the acini from cancer-containing breasts than in the control group, although the ratios for ducts were similar. The reduction in apoptosis may lead to the preservation of genetically aberrant cells, hence favouring neoplastic development. There is a need for further investigation of 'at-risk' cases, including women with a family history, and for a prospective study of a large group of women. 1998