Diabetes mellitus patients must concentrate on self-care, with complex treatments and adequate health behavior in which such habits are a key factor. Due to the complexity of this issue, the importance of literacy in health arises. The goal of the study was to analyze factors associated with literacy in health and its relation with glycemic control in diabetic patients. It involved a study with 82 type 2 diabetic patients of both sexes and aged between 19 and 59 attended in the outpatient endocrinology ward of a public hospital, who filled out an abbreviated and translated version of the Test of Functional Health Literacy in Adults (b-TOFHLA). Fasting glycaemia values and glycated hemoglobin were collected from the clinical history of the participants. Correlations, mean comparisons and linear regression models were tested. Inadequate literacy in health was encountered in 65.9% of the patients. The issues involved factors associated with the b-TOFHLA point scores were age and years of schooling. Global literacy did not explain the glycemic control, but the level of numeracy presented an association with this control. The results point to the need to improve the numeracy in health of the patients to obtain enhanced glycemic control, mainly in those with more advanced age and less years of schooling.