There is lack of research on the importance of including fathers in parent training programs. This study’s main purpose is to examine the short-term effects of fathers attending an Incredible Years Parent Training (IY) for Portuguese preschoolers with oppositional/defiant symptoms. Thirty-six children (whose fathers were willing to attend a parenting group with their wives or partners) were randomly assigned either to receive the IY Program or to a waiting-list control group. Outcomes for the study included self-reported parenting-related variables and parents’ ratings of their children’s behaviors. Data were collected before the intervention and six months after it. Results showed significant effects on fathers’ positive parenting practices and ratings of children’s prosocial behaviors, as well as a reduction of the impact of symptoms on family functioning. Findings provided support for the short-term effectiveness of the IY intervention in Portuguese fathers of preschoolers with oppositional symptoms.