Comparisons between the Danish and Swedish Cancer Registry revealed a 9% difference in 5-year survival for breast cancer patients diagnosed between 1983 and 1989. The purpose of this study was to determine whether previous differences in survival and stage still exist or whether the Danish figures approach those of the Swedish registry. Complete population-based cohorts of patients with breast cancer in the years 1996-1997 from well-defined areas in Denmark and Sweden were compared. The study regions were a Danish (Funen) and a Swedish (Malmö) county with mammography screening and two Danish counties without screening. No difference in extent of disease or survival was observed between screening areas regardless of country. However, there were significant differences in stage distribution and survival between screening populations and Danish non-screening populations, to the benefit of populations provided with a screening programme. Five-year survival was 5-6%, higher in screening populations than in Danish non-screening counties. Corresponding disease-specific survival enhanced the difference. In a multivariate analysis increasing age, tumour size and stage decreased survival. Adjusting for these factors eliminated differences in survival according to country/county. Survival difference could be attributed to early diagnosis and favourable stage in populations offered mammography screening and was not related to nation. Denmark may, through early detection, approach the beneficial stage distribution and survival observed in Sweden.