Vitamin D plays an important role in human health. Current recommendations for vitamin D intake and endogenous supply through sun exposure are not met in German pre-school children, and suboptimal serum 25-hydroxyvitamin D concentrations, especially during the winter months, are common. Consequently, vitamin D supplementation or fortification have gained increased acceptance. The KiMi trial (Kindermilch=growing up milk) was a prospective, randomized, and double-blind study in which young children (2-6 years of age, n=92) were assigned to receive either vitamin D-fortified growing up milk (2.85 μg/100 ml) or semi skimmed cow's milk without added vitamin D. Daily consumption of fortified growing up milk contributed to the prevention of an otherwise frequently observed decrease in serum 25-hydroxyvitamin D concentration during winter (before winter: median 21.5 ng/mL (10.1-43.0 ng/mL) intervention vs. median 18.4 ng/mL (11.0-44.9 ng/mL) control; after winter: median 24.8 ng/mL (7.0-48.2 ng/mL) intervention vs. median 13.6 ng/mL (7.0-36.8 ng/mL) control) and proved to be safe during summer (median 27.6 ng/mL (18.8-40.5 ng/mL) intervention vs. median 27.4 ng/mL (17.8-38.7 ng/mL) control). Due to the high prevalence of vitamin D deficiency, fortification of growing up milk with vitamin D at a level used in this study could be an effective measure to improve vitamin D status.