Objectives:  A study was carried out to determine the reliability, validity and cut-off points of the Spanish version of Ages & Stages Questionnaires: Social-Emotional, 2nd Edition (ASQ:SE-2). Material and Methods: Questionnaire forms of ASQ:SE-2 between 15 to 48 months were applied to parents of children chosen by convenience sampling. Reliability was determined by the internal consistency through Ordinal alpha coefficient (α), and validity of content through Item-test correlation (r). The cut-off points to establish risk zones were determined using the 65th percentile and ROC curve. Results: Two hundred thirty-six questionnaires were applied. The Ordinal alpha coefficient reached a between acceptable to good level (α = 0.73-0.87) for all 5 age-intervals questionnaires between 18 to 48 months. The Item-test correlation index pointed a good discriminatory power for most i...