The Ages and Stages Questionnaire Third Edition (ASQ-3) is a screening test used to detect developmental delays in pre-school children. Despite being translated for many languages such as Spanish, French, Danish, Dutch, Korean and Chinese, ASQ have not been translated to Brazilian Portuguese or studied in Brazil yet. In this study, Psychometric properties of ASQ-BR were examined to understand the effectiveness of the instrument in public child care centers system. Back-translation was used and, cultural adaptations were made to assure the full adaptation of the test to Brazilian culture. The final version of ASQ-BR was used in 45,650 children in public child care centers from Rio de Janeiro. Classical Test Theory (CTT; Cronbach’s alpha and item-to-total correlations), and Item Response Theory (IRT) based on Mokken analysis (Loevinger’s H and MS rho) and Rasch modeling analysis (infit and outfit mean squares, and person and item reliability) were used. Cronbach’s coefficient alpha ranged from 0.52 to 0.89, and item-to-total correlations revealed only 50 of 570 items (8.8 per cent) under 0.30. Mokken analysis corroborated with CTT indexes. In Rasch analysis, only 53 of 570 (9.3 per cent) items showed misfit problems, but no item presented infit higher than 2.0. Person reliability ranged from 0.34 to 0.62 and item reliability ranged from 0.83 to 0.99. Overall, the instrument showed good psychometric properties. The results and the usage of ASQ-BR in public child care system are discussed.