The impact of behavior problems on kindergarten readiness is not known. Our objective was to estimate the association between behavior problems and kindergarten readiness on a US national sample. In the US educational system, kindergarten is a natural point of entry into formal schooling at age 5 because fewer than half of the children enter kindergarten with prior formal preschool education. Parents of 1,200 children who were scheduled to enter kindergarten for the first time and were members of the Harris Interactive online national panel were surveyed. We defined behavior problems as an affirmative response to the question, "Has your child ever had behavior problems?" We validated this against attention deficit hyperactivity disorder diagnosis, scores on a reliable socioemotional scale, and child's receipt of early intervention services. We used linear, tobit, and logistic regression analyses to estimate the association between having behavior problems and scores in reliable scales of motor, play, speech and language, and school skills and an overall kindergarten readiness indicator. The sample included 176 children with behavior problems for a national prevalence of 14% (confidence interval, 11.5-17.5). Children with behavior problems were more likely to be male and live in households with lower income and parental education. We found that children with behavior problems entered kindergarten with lower speech and language, motor, play, and school skills, even after controlling for demographics and region. Delays were 0.6-1 SD below scores of comparable children without behavior problems. Parents of children with behavior problems were 5.2 times more likely to report their child was not ready for kindergarten. Childhood behavior problems are associated with substantial delays in motor, language, play, school, and socioemotional skills before entrance into kindergarten. Early screening and intervention is recommended.