Introduction: Breastfed infants under one year of age may not get enough vitamin D; therefore a vitamin supplement is needed. The adherence to this policy has not yet been evaluated in Chile. Objective: To evaluate the adherence to vitamin D supplementation in children less than one year old and the determinant factors involved. Patients and Method: A cross-sectional study was carried out in three Catholic University Health Network centers. Breastfed infants under one year of age were included in the study. Their parents/guardians filled out a questionnaire about adherence to supplementation and its determinant factors. Results: 170 infants were recruited. 164 of them received supplementation, with a good adherence of 68.9%. The main reason for non-adherence was due to maternal forgetfulness. The identified risk factor for poor adherence was the number of maternal children. Conclusions: Vitamin D supplementation in Chile reaches high levels, but its adherence is poor. More education to parents on ways to avoid forgetting the supplement is needed as well as on identifying risk factors during medical consultations.