The present report is a case control study aimed to determine the levels of cytokines and other parameters in the sera of allergy-complicated and uncomplicated giardiasic children before and after metronidazole treatment. The study included a total of 126 subjects; 52 giardiasic children, 34 allergy-complicated giardiasis (36.9%) and 34 healthy controls, as well as six cases of giardiasis simultaneously infected with other parasites or bacterial pathogens. Serum levels of tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-1beta, soluble IL-2 receptor (sIL-2R), IL-6, IL-8, nitric oxide (NO), and C-reactive protein (CRP) were determined. TNF-alpha and sIL-2R levels significantly increased in giardiasic cases. IL-1beta, IL-6, IL-8, CRP, and NO levels increased only in the cases associated with allergy. All increased variables significantly decreased following metronidazole treatment and returned to normal levels. Metronidazole-treated patients became 100% parasite free. In conclusion, increased TNF-alpha and sIL-2R may be involved in pathogenesis of non-allergic giardiasis and probably Th1 type immune response seems to be predominant and this response may be protective rather than causative of the disease. Activation of the immune system takes place in giardiasis. It is broader and more intense in allergy-complicated giardiasis than that of uncomplicated cases, most probably due to non-invasive character of G. lamblia. Enhanced IgE production pointed to Th2-type immune response and confirms its association with allergy.