Genetic and environmental influences on problem behaviors were studied in 3-year-old twins. Fathers' and mothers' ratings of problem behaviors in twins-236 monozygotic (MZ) girls, 210 MZ boys. 238 dizygotic (DZ) girls, 265 DZ boys, and 409 DZ opposite sex pairs-were obtained with the Child Behavior Checklist for Ages 2-3 (T. M. Achenbach, 1992). Twin correlations and results from a model fitting approach showed that genetic, shared environmental, and nonshared environmental influences accounted on average for about 64%, 9%, and 27% of the variance. Although shared environmental influences were small for most scales, they were important for Total Problems and somewhat larger for Externalizing than for Internalizing behaviors. Significant sex differences in genetic and environmental influences and evidence for sibling contrast effects were found for the Overactive scale.