This study examined the influences of problem behaviors on kindergarten reading outcomes and investigated the extent to which explicit, code-based reading instruction moderated those relations among 206 children identified as being at risk of reading difficulty. Children from the classrooms of 57 kindergarten teachers in 12 schools were randomly assigned to an early reading intervention program or school-determined comparison intervention. In both conditions, children received 30 minutes of small-group supplementary reading instruction for 21 weeks. Findings from multilevel modeling revealed many associations between problem behaviors and reading outcomes for children in both conditions. Interaction analyses indicated that explicit, code-based reading intervention moderated the negative impact of externalizing problem behavior on end-of-kindergarten measures of alphabet knowledge, phonemic blending, and word reading. This type of intensive systematic reading instruction also moderated the influence of hyperactivity on children's alphabet knowledge and phonemic blending. There were no moderator effects for internalizing problem behavior. (Contains 6 tables and 1 figure.)