The transmission route of giardiasis not yet understood and why some infected individuals remain asymptomatic while others become quite ill. The drinking water quality is supposedly responsible for the prevalence of asymptomatic Giardia duodenalis infections in different areas, therefore asymptomatic giardiasis has been investigated in three water supply areas of Hungary: three hundred stool samples from inhabitants of Budapest, Füzér and Mátrafüred were examined by immunological and molecular methods for the presence of G. duodenalis infections. Individuals were asked to fill out a validated questionnaire at the time of stool collection and the interview covered demographic data, family life, education and travel history.In Budapest and in Mátrafüred in one stool sample G. duodenalis Assemblage A, whereas in Füzér once G. duodenalis Assemblage A, once Assemblage B and twice mixed infection were detected. We found higher prevalence rate of 4% of G. duodenalis infections of asymptomatic people in the village Füzér, where the removal of the Giardia cysts of the drinking water treatment plant was not effective. This study throws a light the need to look into the possibility of other risks of Giardia infections such as water transmission routes. To our knowledge, this is the first study evaluating the prevalence of G. duodenalis infections in asymptomatic persons in Hungary.