Background: The Strengths and Difficulties Questionnaire (SDQ) is widely used to screen for child mental health problems and measure common forms of psychopathology in 4- to 16-year-olds. Using longitudinal data, we examined the validity of a version adapted for 3- to 4-year-olds. Methods: We used SDQ data from 16 659 families collected by the Millennium Cohort Study, which charts the development of children born throughout the United Kingdom during 2000-2001. Parents completed the preschool SDQ when children were aged 3 and the standard SDQ at ages 5 and 7. The SDQ’s internal factor structure was assessed by using confirmatory factor analysis, with a series of competing models and extensions used to determine construct, convergent, and discriminant validity and measurement invariance over time. Predictive validity was evaluated by examining the relationships of age 3 SDQ scores with age 5 diagnostic measures of attention-deficit/hyperactivity disorder, autism spectrum disorder/Asperger syndrome, and teacher-reported measures of personal, social, and emotional development. Results: Confirmatory factor analysis supported a 5-factor measurement model. Internal reliability of subscales ranged from omega = 0.66 (peer problems) to omega = 0.83 (hyperactivity). Item-factor structures revealed measurement invariance over time. Strong positive correlations between ages 3 and 5 SDQ scores were not significantly different from correlations between age 5 and 7 scores. Conduct problems and hyperactivity subscales independently predicted developmental and clinical outcomes 2 years later. Conclusions: Satisfactory psychometric properties of the adapted preschool version affirm its utility as a screening tool to identify 3- to 4-year-olds with emotional and behavioral difficulties.