The association with breast cancer of menstrual and reproductive events, family history of breast cancer, and body size have been studied on two cohorts of 6,706 volunteers on the island of Guernsey (United Kingdom), 168 of whom had breast cancer detected during follow-up. The median follow-up time of the non-cases was 21 years in the first study and 10 years in the second. A time-dependent Cox regression model was fitted to the data with age as the time-dependent variable in order to represent the effect of changing menopausal status. Other variables examined in the model were age at menarche, parity, age at first birth, family history of breast cancer, height, weight (both directly measured), relative weight (weight [kg]/height[m]), and Quetelet's body mass index (weight[kg]/height[m]2). Interactions between age and all other covariates also were examined. Family history was found to be the most important risk factor for women aged less than 51 years (relative risk [RR] = 3.5, 95 percent confidence interval [CI] = 2.0-6.0), and intervals between menarche and first birth longer than 14 years were found to increase significantly the risk of breast cancer in women older than 61 years (RR = 2.4, CI = 1.3-4.4). Height was the only indicator of body size which was associated significantly with risk of breast cancer, the estimated regression coefficient indicating an increase in risk of about 70 percent for women on the 90th centile of height relative to those on the 10th centile. A survey of the literature showed that the association between risk of breast cancer and height was found in those studies which used direct measurements of height but not in others which used self-reported values.