Background Regarding the short‐ and long‐term benefits of early intervention for childhood developmental disorders, this study was carried out to determine the prevalence of undetected developmental delays by the standardized form of the Persian version of the Ages and Stages Questionnaires (ASQ) in Iranian children. Methods This was a cross‐sectional study. The study was carried out on 11 000, 4‐ to 60‐month‐old children; in 19 age groups, in 41 selected cities throughout the country. Ninety physicians were invited from each of the selected cities to Tehran in groups, and trained about the ASQ scoring, during a 2‐day workshop. The questionnaires were completed at healthcare settings by parents of children who were apparently normal (lacking any gross developmental disorders) under the supervision of the physicians who consequently scored each child. The response rate was 95% (10 516 parents). Results Among the 10 516 children studied, 5035 (47.87%) were girls. The average percentage of children delayed in the communication, gross motor, fine motor, problem‐solving and social‐personal domains was 3.87%, 4.04%, 4.31%, 4.15% and 3.69%, respectively, when considering the Iranian cut‐off points. In the 19 age‐domain groups, most frequently in the personal‐social and fine motor domains (in terms of domains), and also most frequently at 36 and 48 months of age (in terms of age ranges), girls showed significantly higher scores than boys. Boys showed significantly higher scores in two age‐domain groups (gross motor domain at 20 and 22 months of age). Conclusions The prevalence of undetected developmental delays in Iranian children screened by the ASQ questionnaires ranged from 3.69% to 4.31% in different developmental domains.