Evidence supporting the internal structure of an assessment can provide support for its scoring mechanisms. A total of 1,691 young children in the United States were assessed by their parents using the 16-month questionnaire of the "Ages and Stages Questionnaires," Third Edition (ASQ-3), and results were used to study the internal structure and dimensionality of the ASQ-3 using item response theory (IRT)/Rasch models. Data were analyzed using IRT including unidimensional and multidimensional Rasch Partial Credit Models to examine four theory-based internal structures. Results supported the hypothesis that the ASQ-3 measures a unidimensional construct--the overall development of children--while preference was given to a more precise multidimensional construct of five interrelated domains. However, concerns emerged regarding the extremely high correlations between items in the fine-motor and problem-solving domains. Findings provide implications for the test developers in terms of item refinement to improve measurement of underlying constructs.