Nationwide cancer survival statistics published in Finland and Norway were compared. The results showed that there are cancer sites (e.g. stomach and lung) at which the 5-year relative survival rates are equal in both countries, but also many sites at which the survival figures differ (e.g., cancer of the breast and melanoma of the skin). Ideally, the differences in cancer survival between these countries could be used to study the effects of any differences in health care systems, (e.g., modes of treatment). Differences in cancer survival might also be used in the interpretation of cancer incidence figures. It was shown, however, that even with Finland and Norway, in which cancer registration and follow-up systems are relatively comparable and definitions and classifications criteria continually monitored, several factors complicate survival comparisons. These include distribution by histological type, exact location of the tumours, definitions and distributions of tumour stages, and finally definition of the cancer itself.