In order to study fathers' knowledge of breast feeding and its relationship with paternal factors, fathers of 92 breast feeding and 89 non-breast feeding newborns were compared. Paternal factors included previous children and the way they were fed, participation in prenatal care, attendance at prenatal classes, breast feeding information provided by health professionals, use of reading materials and interest in learning more about the subject. The results indicated that fathers had poor knowledge about breast feeding, especially those whose children were being bottle fed. After adjustments for confounders, fathers who had previous breast-fed child(ren), had attended prenatal classes and who received information about breast feeding from medical personnel had a significantly higher chance of having a better knowledge of breast feeding. It seems that fathers need to be better prepared to assume their new role as breast feeding supporters. Prenatal care was shown to be a good opportunity to improve fathers' knowledge of breast feeding.