We investigated the effectiveness of the Family Check-Up (FCU) and an Internet-based parent-training program (iComet), along with moderators and mediators of outcome. Families (N = 231) with a child with conduct problems were randomized to one of the conditions for 10 weeks of treatment. The drop-out rate was significantly higher in the iComet (39%) compared to FCU (23%). At post-treatment, both conditions resulted in significant improvement, based on parent-report, but no significant interaction between time and condition, with the exception of conduct problem subscale of the Strengths and Difficulties Questionnaire, slightly favoring the FCU. Neither child, nor teacher reports indicated any significant changes on any of the investigated variables. At 1-, and 2-years follow-up, the gains from the treatment were maintained in both conditions, with basically no significant time X condition interactions. A significantly larger proportion of children in the FCU recovered at post-treatment with regard to opposition defiant behavior, inattention, and conduct problems, compared to the iComet, but almost none of these differences remained significant at 1-, and 2-years follow-up. None of the moderators (child age, parental income or education, or pre-treatment level of motivation) or mediators (limit setting, and appropriate or harsh parenting) of outcome turned out to be significant.