The overall death rates are highest in the winter season in many countries at high latitudes. In some but not all countries, this is also true for more specific diseases such as cancer, cardiovascular diseases and influenza. For internal cancers we find no consistent, significant seasonal variation, neither of incidence nor of death rates. On the other hand, we find a significant seasonal variation of cancer prognosis with season of diagnosis in Norway. Best prognosis is found for summer and autumn diagnosis; i.e., for the seasons of the best status of vitamin D in the population. There were no corresponding seasonal variations, neither of the rates of diagnosis, nor of the rates of death which could explain the variations of prognosis. The most likely reason for this variation is that the vitamin D status in Norway is significantly better in summer and autumn than in winter and spring. Earlier, seasonal variations have been explained by circannual variations of certain hormones, but the data are not consistent.
Keywords: cancer; incidence; prognosis; seasonal variation; vitamin D.