Giardiasis is a parasite illness of alimentary canal, caused by invasion of primeval – flagellum from species of Giardia intestinalis. Infection appears all over the world, in some areas it affects up to 30% of population. It affects mostly children of 2-5 years old. The infection passes from the infected persons. Life cycle of Giardia takes place in one individual and embraces two stages of development: trofozoit and cyst. Alimentary canal is a way of infection which can be caused by drinking water or eating food containing lamblia’s cysts. In most cases it affects children gathered in kindergartens or day nurseries where the infection is spread by direct contacts. Clinical outcome of giardiasis is varied, it can present either no symptoms or a typical but not very specific symptoms, e.g., diarrhoea, stomachache or fever. The disease can cause chronic diarrhoeas, the lost of weight, and give rise to the symptoms of allergy. Finding the cysts in stool is diagnostic but plenty of examines are usually necessary. The immunoflourescent examination and immunoenzimatic technique (ELISA) become commonly used in detection of antigen Giardia lamblia. These testes display high sensitivity and specificity. Such drug substances as metronidazole or other nitroimidazoles has been used for years and they are still the first choice in treatment of G. intestinalis infection. In this paper two cases with different clinical outcome are presented. Both cases turned out to be a diagnostic challenge. The difficulties delayed the application of proper treatment and recovery.