<p>Close to 15% of children adopted out of U.S. foster care face challenges, including post adoption instability and strained family relationships. This study examined how clinical scores on the Child Behavior Checklist Problems (CBCL) total, externalizing, and internalizing scales/subscales were related to post adoption instability—either formal foster care reentry or informal placement changes. 383 adoptees and/or adoptive parents were surveyed as part of the NSCAW Adoption Study. Using data from this survey, bivariable analyses and multivariable regression models with propensity score weighting estimated the impact of clinical versus nonclinical child behavioral challenges on instability, accounting for adoptee age at adoption, race, gender, and sexual abuse history; parent-child kinship; and parent age at adoption. Bivariate analyses indicated that 44% of adoptees with clinical CBCL total scores experienced post adoption instability, as compared to 23% of adoptees who never had clinical CBCL total scores, with similar patterns found for the externalizing and internalizing subscales. Multivariable models with propensity score weights showed that having a clinical score on the CBCL total, externalizing, or internalizing scale/subscale was associated with higher risk of post adoption instability than those without clinical CBCL scores, after controlling for covariates. An older child age at adoption and history of sexual abuse were both positively associated with clinical CBCL scores. Findings highlight the importance of focusing risk assessment and intervention efforts on families who have the most difficulty managing children’s behaviors. Further, child welfare agencies that work with adoptive families should regularly assess child behavior to prevent post adoption challenges.</p>