The survival of colon cancer patients in Norway, as determined three years after diagnosis, is dependent on the season of diagnosis. This has been attributed to seasonal variations of the vitamin D status. Since solar radiation and food are the human sources of vitamin D, we divided Norway in three regions: The southeast region with a high annual dose of ultraviolet (UV) to the population, as evidenced by a high incidence rate of squamous cell carcinoma of the skin (SCC), the midwest region and the north region with low annual UV doses. The latter region is characterized by a high consumption of vitamin D, mainly through fish intake. Vacations to southern latitudes were equally frequent for all the three geographical regions. Two age groups were analyzed separately (< or =65 years and >65 years), since the photosynthesis of vitamin D(3) in skin decreases with age. In all three regions, and in both age groups, the survival was highest for summer and autumn diagnosis. The seasonal effect was slightly, but not significantly, better for the younger than for the older age group. The effect was similar for all three geographical regions, irrespective of SCC incidence.