There is a four to fivefold variation in breast cancer incidence rates across different countries. The lowest rates are observed in Asia, and the highest rates are observed in western Europe and North America. The incidence of breast cancer has increased in all countries since 1960. We studied in detail (whenever possible) the changes in incidence and mortality between 1955 and 1990 in four age groups (35-44, 45-54, 55-64 and 65-74) for 11 "representative" countries (USA, England and Wales, Norway, Hungary, Yugoslavia, Spain, Colombia, Singapore, Japan, India and China). The largest increase in incidence took place in Japan and Singapore. The incidence rate for women aged 35-44 in Japan doubled between 1960 and 1985 and by 1985 was roughly two thirds the USA rate. There has been essentially no change in mortality rates in the USA, England and Wales or Norway, whereas there has been a 50-60% increase in Japan, Singapore and Hungary. Most of the observed increase in incidence rates in the USA, England and Wales and Norway may be "artefactual", that is, due to changes in screening patterns. Screening may also have contributed to the rate increase in other countries, but outside western Europe and North America the major part of the increase is likely to be due to changes in known and suspected breast cancer risk factors.