Qualitative studies suggest that children react to parental imprisonment by developing internalizing as well as externalizing behaviors. However, no previous study has examined the effects of parental imprisonment on children's internalizing problems using standardized instruments, appropriate comparison groups, and long-term follow-up. Using prospective longitudinal data from the Cambridge Study in Delinquent Development, we compared boys separated because of parental imprisonment during their first 10 years of life with four control groups: boys who did not experience separation, boys separated because of hospitalization or death, boys separated for other reasons (usually parental disharmony), and boys whose parents were only imprisoned before the boys' births. Individual, parenting, and family risk factors for internalizing problems were measured when boys were ages 8–11 years. Separation because of parental imprisonment predicted boys' internalizing problems from age 14 to 48, even after controlling for childhood risk factors including parental criminality. Separation because of parental imprisonment also predicted the co-occurrence of internalizing and antisocial problems. These results suggest that parental imprisonment might cause long-lasting internalizing and antisocial problems for children.