Vaccination is the administration of antigen into the body to create antibodies and is a type of active immunization. Vaccinating a large number of people eliminates the disease and ensures collective immunity that protects those individuals who, for health reasons, cannot be vaccinated. Although vaccination has been practiced for many years and is proved to be a successful way of achieving collective immunity, over the last few years there has been a growing negative attitude towards vaccination and the spread of the anti-vaccine movement. Research has found a correlation between a positive attitude toward vaccination and the use of formal sources of information such as doctors or professional literature as well as the correlation of a negative attitude to vaccination with information about vaccines gained from media or from other people. Some studies have found a correlation between formal education level and attitude and knowledge about vaccination. The aim of this study was to examine the relationship between knowledge and attitude of Croatian adults about vaccination and to examine the relation of knowledge and attitude with some sociodemographic characteristics of Croatian adults (age and level of education) and the use of various sources of vaccination information. This study involved 189 participants who completed an online questionnaire. The questionnaire consisted of questions about certain sociodemographic characteristics, questions about the sources of information on vaccination, vaccination attitude questionnaire and the vaccination knowledge test. The results did not show a significant correlation between the attitude and knowledge of Croatian adults about vaccination, nor between the attitude and knowledge of Croatian adults about vaccines and their age. The formal level of education is not significantly related to the knowledge about vaccination, however, there was a statistically significant, albeit low, positive correlation between the level of education and the attitude about vaccination. Regarding the correlation between knowledge and attitude about vaccination with the use of different sources of vaccine information, the results show a low positive correlation between the physician as a source of information and attitudes towards vaccination and between the use of professional literature as a source of information and knowledge on the test about vaccination. Furthermore, there was a low negative correlation between attitudes towards vaccination and the internet / media and other people as sources of information.