Despite extensive studies on ASD, the etiology of this disorder is quite unknown and studies are ongoing [
2,
3]. ASD has a dominant genetic origin. However, environmental and genetic factors have interaction in the incidence of this disorder [
3‐
5]. The results of studies have shown that in disease etiology, risk factors such as prenatal and postnatal infections [
6,
7], and exposure to Valproic Acid of alcohol during pregnancy [
8,
9], the age of mother [
10], and abnormal nutritional and metabolic factors [
3] are effective. During the recent years, the incidence of ADSs has been significantly increased. In previous studies, the incidence of this disorder was 10 in 10,000 [
11], whereas the incidence of this disorder is now estimated as 90–250 in 10,000 [
12‐
15]. In addition, in 2010, CDC reported the incidence of autism disorder in the United States as 1 in 68 and this indicates 78% increase in the incidence level compared with 2002 [
16]. However, a part of this sudden increase is probably the result of increased awareness and better reports about autism disorder as well as improved diagnostic criteria, but the exact causes for this sudden increase should be determined in future studies [
17]. Increased incidence of the disease can impose a heave financial burden on the society. It is estimated that medication costs for each patient will be 40,000 to 60,000 dollar per year [
16]. During the past decades, numerous studies were conducted on the role of vitamin D in neuropsychological disorders [
18‐
23]. The findings of these studies showed that vitamin D deficiency is one of the risk factors of evolutional neuropsychological disorders such as schizophrenia [
24] and autism [
19,
25‐
28]. However, studies on the relationship between vitamin D and autism in different parts of the world such as Sweden [
29], Egypt [
20], Saudi Arabia [
30], and China [
31,
32] indicate lower 25 (OH) D level in patients with ASD in different ages compared with the control group. Moreover, some studies [
33,
34] have shown different findings and no significant difference was observed between serum levels of vitamin D in ADS and control groups. To our knowledge few studies have been conducted in this regard in Iran and no study has been conducted in Ahvaz city, Southwestern Iran. Therefore, the present study aimed to evaluate the serum 25 - hydroxyl vitamin D level among children with ASDs in Ahvaz city, Iran.