The serum 25-hydroxy-vitamin D (25-OH-D) concentrations of a group of healthy persons (age 30-50 years) living north of the Arctic Circle in Finland was compared with those of a group of healthy persons (age 30-50 years) in Southern Finland throughout one year. There was a similar seasonal variation in the 25-OH-D levels both in the south and in the north. The levels were significantly higher in the north than in the south in the winter (p less than 0.01) and the autumn (p less than 0.05). The samples in the south were, however, taken 11/2 month later than in the north. There was no seasonal variation in the serum calcium or the serum parathyroid hormone levels in the north and the serum parathyroid hormone levels did not differ from that in the south. The results indicate that the vitamin D status of healthy young people living in the north is satisfactory and comparable with that of healthy young adults in the south of Finland.