Age-standardised mortality rates for breast cancer were examined for 20 countries in Europe, North America, Australia and New Zealand from 1950 to 1992 and age-birth cohort and age-period of death models were fitted to the data. Breast cancer mortality rates generally increased in the earlier decades, but more recently rates have levelled off or begun to decline in most countries. Only in 4 of the 20 countries studied, Belgium, Hungary, Poland and Spain, was there no evidence of a decline or leveling off or mortality in recent birth cohorts or in recent years. In the other countries the decline in mortality appeared to be in part due to birth cohort effects and in part due to period effects. The birth cohort effects were suggestive of a decline in breast cancer rates among women born after about 1920 and were evident in many countries especially Canada, The Netherlands, The United Kingdom and the United States. The decline in mortality in women born after 1920 appeared to be in part related to a reduction in childlessness and a reduction in age at first birth in those generations. As well as the birth cohort effects, there was some evidence of a recent overall decline in mortality rates in several countries, e.g. Austria, FRG, Greece and the UK, and this may be due to an increase in survival resulting from improved management and treatment of women with breast cancer.