Breast cancer is the most common cancer in women worldwide (Parkin, 2001). In England and Wales at the end of the 1990s, approximately 36 000 new cases were diagnosed each year, representing 30% of the annual total of 120 000 cases. The number of cases exceeds the combined total for the second and third most common cancers in women, those of the large bowel (14 700 cases, 12%) and lung (12 600 cases, 10%). Breast cancer in men is comparatively uncommon, and is not considered further here. During the 1970s and 1980s, age-standardised incidence increased on average by approximately 2% each year (Quinn et al, 2001). The NHS breast-screening programme began in 1988, covering women aged 50–64 years and using single mediolateral oblique view mammography and a 3-year interval between screens. During the prolonged prevalence round, overall breast cancer incidence increased by approximately 20%; it subsequently declined, but then rose again, and by the late 1990s, incidence was some 7% higher than at the peak in the early 1990s (Office for National Statistics, 2003). Part of the increase, possibly as much as 20% in post-menopausal women, may have resulted from the increasing use of hormone replacement therapy (HRT) in the 1990s (Beral et al, 1997). After the exclusion of in situ tumours, we analysed the data for 382 277 women diagnosed with a first, primary, malignant neoplasm of the breast in England and Wales during the 14-year period 1986–1999 and followed up to 31 December 2001, some 89% of those eligible for analysis. Approximately 6.1% of women who were otherwise eligible were excluded with zero recorded survival (date of diagnosis same as date of death): most of these women will have been registered from a death certificate only (DCO), and their duration of survival is unknown, but they could not be reliably distinguished in these data from women with true zero survival. The proportion of women excluded from analysis as DCO records was very similar in all socioeconomic groups. A further 2.4% of women were excluded because it was not their first primary cancer: for one-third of these women, the previous cancer was also a breast cancer, diagnosed before 1986. Major shifts in the morphological distribution of breast cancers have occurred over the last 30 years. The proportion described as an adenocarcinoma has been falling steadily, from 36% in the early 1970s (Coleman et al, 1999) to 6% in these data by the late 1990s. In the early 1970s, ductal, lobular and medullary tumours comprised less than 10% of all breast cancers, but by the late 1990s this figure had reached 75%, ductal carcinomas alone comprising 60% of all breast cancers. The proportion of tumours with unspecified morphology has continued to fall, from 47% in 1971–1975 to 30% in 1986–1990 and less than 5% in 1996–1999. Even this massive improvement in data quality cannot account for the overall increase in ductal, lobular and medullary tumours.