The principal source of human natural vitamin D is cholecalciferol produced in skin by the action of ultraviolet light. Plasma levels of 25-hydroxyvitamin D (25-OH-D), the hepatic metabolite of cholecalciferol, increased by 39.3% in 20 healthy young Swiss subjects after a 3-week holiday in the sun. Significant increases of circulating 25-OH-D (p smaller than 0.001) were found as much as 2 months later when compared with a group not deliberately exposed to sunshine. A short vacation in the sun therefore appears to be beneficial in replenishing vitamin D body stores.