Time trends and differentials in cancer incidence in the five Nordic countries, Denmark, Finland, Iceland, Norway and Sweden, were investigated, using material collected by the cancer registries in each country. The incidence at all sites combined and at 23 anatomical sites was studied by age, birth cohort and time period. The maximum lengths of the trends were used for each country. In Denmark the material comprised all the tumours diagnosed in 1943-1980, in Finland and Norway those diagnosed in 1953-1980, in Iceland those diagnosed in 1955-1980, and in Sweden those diagnosed in 1958-1980. For males the age-adjusted cancer incidence rates at all sites combined were highest in Denmark and Finland, and lowest in Sweden and Norway. In females the incidence was highest in Denmark and Iceland, and lowest in Finland. The rates increased slightly for both sexes. For cancer of the pancreas, Hodgkin's disease, acute leukaemia and childhood cancer (all sites combined) the rates in all the Nordic countries were similar every year. For cancers of the stomach, colon, breast, corpus uteri, ovary, prostate, testis, urinary bladder, melanoma of the skin and non-Hodgkin's lymphomas the trends were similar but on different levels. For cancers of the larynx and lung in males the rates in Finland decreased during the 1970s, whereas the rates were increasing in the other Nordic countries. For cancer of the rectum, the trend showed a decrease in Denmark but an increase in the other Nordic countries. For lip cancer the rate in Sweden was almost constant over time, but in Denmark, Finland and Norway a decrease occurred. For oesophageal cancer in males the rates decreased in Finland and Iceland in the 1970s, whereas in Denmark and Norway there was very little change, and in Sweden there was an increase in the rates. For cancer of the cervix uteri the rates started to decrease in Denmark, Finland, Iceland and Sweden in the mid-1960s, but in Norway not until some ten years later. The differentials between the countries were largest for cancers of the testis and thyroid, in which the highest incidence was five to six times as large as the lowest. For testicular cancer the rate was the highest in Denmark, for thyroid cancer in Iceland. For both of these cancers the rate was the lowest in Finland. Melanoma of the skin was the cancer with the most rapid increase in incidence with time in all the Nordic countries.(ABSTRACT TRUNCATED AT 400 WORDS)