Abstract Background This study assessed the concurrent validity of the Ages and Stages Questionnaire (ASQ) compared with Bayley Scales of Infant Development II (BSID II) amongst children aged 24 months. Methods Data were collected from 53 infants and mothers who participated in the New York State Angler Cohort Child Development Study. Parents completed the 24-month ASQ to assess communication, personal-social, problem-solving ability, and fine and gross motor control. The BSID II was administered by a clinical psychologist at the 24-month home visit for cognitive and psychomotor assessment. The ASQ was scored using age-specific norms of <2 SDs below any domain mean to define failure. A BSID II score of <85 indicated mild or severe delay, while a score of <70 suggested a severe delay. Results Scores on the ASQ communication and personal-social domains were moderately correlated with the BSID II Mental Scale (R= 0.52, P < 0.001; R= 0.45, P < 0.01) and ASQ gross motor with the BSID II Motor Scale (R= 0.46, P < 0.01), whereas ASQ problem-solving and fine motor domains were not significantly correlated with BSID II scores. The ASQ had a sensitivity of 100% and specificity of 87% at 24 months (n= 40) for severely delayed status. Conclusions Results suggest the ASQs provide a simple, valid, and cost-effective method for clinicians and field-based researchers to reduce the number of standardized assessments required to identify developmentally delayed infants at age 24 months. Future studies should further assess the validity of the ASQs in larger, more diverse populations of infants.