Background: Little is known about how best to implement behavioral screening recommendations in practice, especially for children in foster care, who are at risk for having social-emotional problems. Two validated screening tools are recommended for use with young children: the Ages and Stages Questionnaire: Social Emotional (ASQ-SE) identifies emotional problems, and the Ages and Stages Questionnaire (ASQ) identifies general developmental delays in five domains, including personal-social problems. The current study examined: (1) whether systematic use of a social-emotional screening tool improves the detection rate of social-emotional problems, compared to reliance on clinical judgment; (2) the relative effectiveness of two validated instruments to screen for social-emotional problems; and (3) the patterns of social-emotional problems among children in foster care. Methods: We used retrospective chart review of children in foster care ages 6 months to 5.5 years: 192 children before and 159 after screening implementation, to measure detection rates for social-emotional problems among children. The ASQ-SE and the ASQ were used in multivariable logistic regression analyses to examine associations between children with social-emotional problems. Results: Use of the screening tool identified 24% of the children as having a social-emotional problem, while provider surveillance detected 4%. We identified significantly more children with social-emotional problems using the ASQ-SE than using the ASQ, and agreement between the instruments ranged from 56% to 75%, when data were stratified by age group. Multivariable modeling showed that preschool children were more likely to have a social-emotional problem than toddlers and infants (aOR = 3.4, 95% CI = 1.1-10.8). Conclusions: Systematic screening using the ASQ-SE increased the detection rate for social-emotional problems among young children in foster care, compared to provider surveillance and the ASQ. A specific social-emotional screening tool appears to detect children with psychosocial concerns who would not be detected with a broader developmental screening tool. (Contains 3 tables.)