Acute respiratory tract infections (ARTIs) are associated with high levels of morbidity and socioeconomic impact, particularly affecting young children. Observational studies have shown an association between vitamin D (VD) deficiency and higher incidence and severity of respiratory infections. Chile has a high prevalence of VD deficiency, particularly in the southern regions of the country. The objective of this study is to evaluate the safety and efficacy of oral VD supplementation to decrease the incidence of ARTIs in 275 preschool children. A randomized, placebo-controlled, double-blind study will be performed in Santiago, Coyhaique and Punta Arenas. Children aged 18 to 36 months will be given weekly oral 5600 IU of vitamin D3 (VD3), 11200 IU of VD3, or placebo doses during 6 months.