At the beginning of the 3rd millennium, one of the most important wishes of the human being is to prevent the causes that disturb the intelligence, especially in the childhood. One of the most famous disorders that seems to have such an effect, is Obstructive Sleep Apnea Syndrome (OSAS), however, only a few studies have described the level of intelligence or cognition in the children with OSAS. Because of finding no preliminary study to search for any relation between the intelligence and chronic nasal or nasopharyngeal obstruction leading to chronic mouth breathing in the literature, the authors arranged a study to evaluate this relation. In a nonrandomized cross- sectional study, 60 otherwise healthy children with 6-12 years of age whom had been referred to the clinic of otolaryngology of Al- Zahra hospital in Isfahan were selected as the case group with the chief complaint of mouth breathing for at least 2 months. Besides, we selected 60 healthy children from 2 schools in Isfahan as the control group that had no history of chronic mouth breathing during last 6 months. All cases were referred to the clinic of psychiatry to test their intelligence. The data gathered and analyzed using T-Test. The study resulted that mean IQ of 60 subjects in the case group, with a range of 89-114, was measured about 100.07± 6.15. Mean IQ of the subjects in the control group was measured about 102.73± 6.14 (range 94-124). There was a statistically significant difference between the mean IQ of the case and control groups; so, there was found a relation between IQ and chronic mouth breathing (P=0.02). It is concluded that the chronic mouth breathing would lead to a lowered IQ comparing with the control group. Some of the most common causes of mouth breathing due to nasal or nasopharyngeal obstruction are adenoid hypertrophy and allergic rhinitis. The treatments of both of etiologies are easily accessible. This means that the lowered IQ of children with chronic nasal or nasopharyngeal obstruction seems to be, at least in some degree, preventable. These initial findings about the relation of IQ and chronic mouth breathing suggest that these are identifiable and preventable sequelae of childhood and that it is very important to evaluate the etiology of this problem and improve the patients' IQ with the appropriate intervention.