The calcidiol level in a group of Norwegians (14,000 individuals, age range 16-80) was found to be highest in late summer. The seasonal variation was larger for young than for old persons. The calcitriol concentration was practically constant throughout the year. Younger persons had less calcidiol and more calcitriol than older persons, indicating that the conversion of calcidiol to calcitriol is more efficient in younger persons. A seasonal variation of prognosis of cancer (colon-, breast-, prostate- cancer and Hodgkin lymphoma) was found. The survival is highest for summer and autumn diagnosis, corresponding to maximal calcidiol levels. Thus, calcidiol may act synergistically with traditional treatment modalities. In view of these calcitriol and calcidiol data, the seasonal variation of cancer survival may be related to the calcidiol gradient, indicating that this Vitamin D metabolite may be more important than believed so far.