This study examined the New Perspectives Aftercare Program (NPAP) for serious juvenile and young adult offenders in The Netherlands. Participants (n = 127) were randomly assigned to NPAP (n = 66) or existing aftercare services ("treatment as usual" [TAU], n = 61). The aim was to determine whether NPAP was effective in decreasing cognitive distortions and criminal thinking patterns and increasing prosocial skills of the juveniles compared with TAU. No direct intervention effects were found on any of the outcome measures. Moderator analyses, however, showed several interaction effects of ethnicity and coping skills for both NPAP and TAU youths. Furthermore, NPAP dropouts displayed significantly more indirect aggression at posttest compared with youths dropping out from TAU. Possible explanations for the mostly null effects are discussed, including implications for further research, policy, and practice.