Giardia intestinalis is an enteric pathogen causing certain digestive disorders by attaching itself on the human intestine and, especially on the duodenum. It is reported that there is a strong relationship between the nitric oxide (NO) level and the clinical prognosis of the infection in infections of G. intestinalis. The aim of the study was to investigate serum NO levels in samples taken from patients with G. intestinalis detected in the microscopic stool examination and from the control group consisting of healthy people and the presence of any relationship with Giardiosis. The study consisted of 60 patients diagnosed with G. intestinalis in microscopic stool examination, [31 (51.7%) male, 29 (48.3%) female] and 60 healthy people [28 (46.7% male, 32 (53.5%) female] as the control group. All patients in the patient group with gastrointestinal symptoms had chronic diarrhea and cysts and trophozoites were observed in microscopic examination of the feces in all of them. Measurement of serum NO level was performed using Cortas method. When the data were evaluated by T-test in the Independent Groups, NO levels in the patient group was found low at a statistically significant level. In Giardiosis, the decline in the NO level suggested that the disease causes a more severe clinical presentation. In addition to this, we consider it necessary that extensive experimental or clinical studies be conducted on the subject.