Background: Accurate and efficient developmental screening measures are critical for early identification of developmental problems; however, few reliable and valid tests are available in Korea as well as other countries outside the USA. The Ages and Stages Questionnaires (ASQ) was chosen for study with young children in Korea. Methods: The ASQ was translated into Korean and necessary cross-cultural adaptations were made. The translated version was then distributed and completed by 3220 parents of young children between the ages of 4 months and 5 years. Reliability was studied including domain correlations, internal consistency, and performance of identification cut-off scores for the Korean population. Rasch analyses including tests of Differential Item Functioning, contrasting Korean and US samples were also performed. Results: In general, internal consistency of the Korean ASQ was high, with overall correlations 0.75 for communication, 0.85 for gross motor, 0.74 for fine motor, 0.72 for problem solving, and 0.65 for personal-social. Validity, including concurrent validity, also had strong evidence. Mean scores of children on the Korean translation of the ASQ and the US normative sample were generally similar. Rasch analyses indicated the majority of items functioned similarly across the Korean sample. Conclusions: In general, the ASQ was translated with cultural appropriateness in mind and functioned as a valid and reliable parent-completed screening test to assist in early identification of young children with developmental delays. Further research is needed to confirm these results with a larger and more diverse Korean sample.