In Spain, studies that investigate parents' beliefs on antibiotic use are lacking. Mistaken beliefs lead to inappropriate antibiotic use and encourage overuse. The aim of the present study was to determine parents' knowledge about antibiotic use. We performed a cross sectional, descriptive study in two health centers through a self-administered questionnaire completed by 348 parents. The response variable consisted of beliefs evaluated when the parents answered the following three statements in the affirmative: a) there are currently some infections with inadequate response to antibiotic treatment; b) if antibiotics are used too often, they can cease to have an effect; c) if your child frequently receives antibiotic treatment, subsequent use may have no effect. Independent variables consisted of sex of the responder, educational level, parental age and ethnicity, attendance at school (public or private), attendance at kindergarten, usual healthcare (public-private), and number of siblings. Statistical analysis consisted of logistic regression. A total of 31.6 % (95 % CI: 26.7-36.5) responded affirmatively to the three statements. Adequate knowledge was associated with the non-immigrant population (OR: 5.7; 95 % CI: 1.37-24.9) and high parental education (OR: 2.04; 95 % CI: 1.16-3.06). Parents' knowledge of antibiotic use is low compared with that in other countries. Health education programs to remedy this situation are urgently required.