This longitudinal study identified hard-to-manage older toddlers and younger preschoolers, assessed the developmental course and prognostic significance of their difficult behavior, and delineated family factors possibly associated with early and persistent problems. Obtained from parents, teachers, and observers, cross-situational and multi-dimensional data concerned 30 boys and 16 girls termed problem children and 11 boys and 11 girls constituting a comparison group. Subjects were assessed at approximately 3, 4, and 6 years of age. Findings indicated that parental complaints concerning toddlers/preschoolers were, for the most part, indicative of real difficulties. Lower social class, greater family stress and disruption, and a negative mother-child interaction were associated with maternal ratings of more severe symptomatology at initial assessment. Follow-up data indicated that children who showed externalizing problems of at least moderate severity in the early preschool years were at a relatively high risk for persistent problems at school entry. Predictors of persistent problems at age 6 included a negative mother-child interaction, higher concurrent levels of family stress and lower social class, and initial maternal ratings of more severe hyperactive and aggressive symptomatology at 3 years of age. (RH)