Background: Controlling, uninvolved, and rejecting parenting in early childhood are strong predictors of later disruptive behavior disorders. However, there have been no evaluations of non-targeted groups for parents of very young children, despite their potential advantages. Methods: We randomly assigned 79 mothers of 12- to 36-month-olds to an 8-session parent training program (called "COPEing with Toddler Behaviour") or to a waiting list control condition. We investigated the immediate and short-term impact on parent-reported child behavior problems, observed parent-child interaction, and self-reported parenting behavior and parent functioning. Results: In an intent-to-treat design, the program yielded significant effects on child behavior problems, positive parent-child interaction, and parental overreactivity and depression but not observed negative child behavior or parental laxness. Most effects were significant at both post-test and 1-month follow-up and effects sizes were small to medium for the intervention group and inverse to small for the control group. Conclusions: The potential of the program to prevent later behavior problems is supported by improvements in six of the eight outcomes. As part of a community strategy, groups such as COPEing with Toddler Behaviour may promote positive parent-child interaction and children's mental health.