The scope of this article is to analyze the understanding of mothers and persons responsible for infants taking ferrous sulfate supplement about anemia and its consequences, assessed by the level of adherence to supplementation. A prospective, qualitative and quantitative population study was conducted with non-anemic children not taking supplements to receive prophylactic supplementation with ferrous sulfate for six months. After six months, there was adherence to supplementation. Among the 133 children initially eligible for the study, 97 returned for the second evaluation. Of these, 4 had anemia during the follow-up and began treatment and 3 others were brought in by individuals who were unable to provide information on use of the supplement. High adherence was showed by 56.7% of children and low adherence by 43.3%, while 23.3% had interrupted supplementation due to the lack of guidance and support of the health service. Mothers and persons responsible of the two groups (high and low adherence) manifested little knowledge about anemia, its prevention and health consequences for the child. More information regarding iron deficiency anemia, routine follow-up by professionals and evaluation of the perception of individuals involved with iron supplementation is necessary.