The efficacy of bibliotherapy has primarily been investigated in anxiety disorders, depression, or substance dependence. The efficacy of self-help books to increase parenting competence was only investigated in a few studies despite their broad dissemination in public. The aims of the study were to investigate the short- and long-term efficacy of a therapist assisted version of the Triple P self-help booklet (Sanders, Markie-Dadds, & Turner, 2003) for families with preschool-age children in Germany. Sixty-nine families were randomly assigned to either a therapist-assisted self-administered parent training (SDPT+T) or to a waitlist control group (WL). Parents in the SDPT+T received the 10 chapter self-help book and an accompanying video. A Triple P facilitator offered seven telephone consultations which aimed to support parents in skill implementation. After the post test, the WL parents were also offered the intervention. A follow-up assessment was conducted six months after post. Compared to waitlist controls, SDPT+T mothers reported significant short- and long-term reductions in child behavior problems as well as in dysfunctional parenting practices. Fathers reported only marginal changes. The study adds further empirical support of parenting self-help materials. (Contains 6 tables.)