Breast cancer is the most frequent malignancy among women in developed countries. Prognosis is better than for other major cancers, and an improvement in survival has been reported for several populations in recent decades. Within the framework of EUROCARE, a population-based project concerned with the survival and care of cancer patients in Europe, we analysed data from 119,139 women diagnosed with breast cancer between 1978 and 1985 in 12 countries and followed for at least 6 years. Multiple regression models of relative survival, which take mortality from all other causes in each area into account, were used to estimate the effect of age, period of diagnosis and country on survival. For the comparison between countries, survival rates were age-standardised to the age structure of the entire study population. Women aged 40-49 years at diagnosis had the best prognosis in all countries and throughout the study period. Women younger than 30 years at diagnosis had a worse prognosis than those aged 30-39. The highest relative survival at 5 years was in Finland and Switzerland (about 74%), intermediate levels were found for Italy, France, The Netherlands, Denmark and Germany (about 70%) and the lowest rates were in Spain, the United Kingdom, Estonia and Poland (55-64%). During the 6 months following diagnosis, survival was highly dependent on age and was sharply lower in women older than 49 years. For women surviving more than 6 months after diagnosis, survival was similar for all ages, although women aged 40-49 still had the better prognosis. The average rate of death from breast cancer fell by about 2.5% for each year of diagnosis between 1978 and 1985. This improvement manifested mainly in younger and older women, for whom survival was initially less good. The largest improvement was seen in Poland (-15% death risk per year). We suggest that the better survival of women aged 40-49 at diagnosis is related to lower levels of circulating sex hormones, resulting in reduced stimulation of tumour cell growth. Early diagnosis may also be important in the peri-menopausal period due to increased diagnostic attention. Low survival in the United Kingdom may be due to inadequate adherence to consensus treatment guidelines and greater variation in treatment.