This descriptive study in the interior of São Paulo aimed to verify diabetes mellitus patients' knowledge about the disease, causes and complications, highlighting its importance in self care. Data were collected through interviews with 84 persons and analyzed through descriptive statistics. Average age was 53.3±13 years, time of disease 12.9±9 years and 58% of the participants did not finish basic education. Only 28.6% of the participants gave correct answers to "what is diabetes" and "what are its causes"; 71% were diagnosed without presenting classic symptoms and 64% had already been hospitalized due to an acute or chronic complication. We indicated aspects that turn the learning process more difficult; little knowledge about the disease, its causes and symptoms, thus affecting the prevention and early diagnosis and entailing predisposition towards complications. Furthermore, the interference of biopsychosocial factors in the self care process is highlighted.