Giardia lamblia has proved to be the most common intestinal protozoan parasite in humans that causes giardiasis. Given the high mutations in the genome of this parasite, the present study was conducted to determine the prevalence of Giardia lamblia subtypes and their relationship with clinical symptoms in patients who appear to have giardiasis. In 69 stool specimens with an appropriate number of giardia cysts, the DNA was first purified, and the genotype was then determined based on the glutamate dehydrogenase (gdh) gene sequence using PCR-RFLP. Data were collected on the clinical symptoms of the patients through a questionnaire, and their relationship with molecular results was studied. Four samples (5.8%) were found to be of subtype AI, 51 (73.9%) of subtype AII and 14 (20.3 %) of subtype BIII. No BIV subtype was found in the present study. A correlation was observed between Giardia lamblia genotypes (AI, AII and BIII) and abdominal pain. All of the people (100%) who had the AI genotype (i.e. the zoonosis subtype) had been losing weight. There was a significant correlation between weight loss and the AI subtype. All of the people (100%) infected with the BIII genotype experienced diarrhea, and this genotype was found to be associated with diarrhea. The present study found abdominal pain to be the most common symptom of giardiasis in Ilam province. Moreover, humans were found to be the main reservoir for giardia lamblia, although zoonosis subtypes such as AI and BIII still exist in the region and pose a risk for a giardiasis epidemic.