In the present study, we used separate measures of parental monitoring and parental knowledge and compared their associations with youths' antisocial behavior during preadolescence, between the ages of 10 and 12. Parental monitoring and knowledge were reported by mothers, fathers, and youths taking part in the Environmental Risk (E-Risk) Longitudinal Twin Study that follows 1,116 families with twins. Information on youths' antisocial behavior was obtained from mothers as well as teachers. We report two main findings. First, longitudinal cross-lagged models revealed that greater parental monitoring did not predict less antisocial behavior later, once family characteristics were taken into account. Second, greater youth antisocial behavior predicted less parental knowledge later. This effect of youths' behavior on parents' knowledge was consistent across mothers', fathers', youths', and teachers' reports, and robust to controls for family confounders. The association was partially genetically mediated according to a Cholesky decomposition twin model; youths' genetically influenced antisocial behavior led to a decrease in parents' knowledge of youths' activities. These two findings question the assumption that greater parental monitoring can reduce preadolescents' antisocial behavior. They also indicate that parents' knowledge of their children's activities is influenced by youths' behavior.