The present randomized controlled trial examined the four months follow-up effectiveness of Parent Management Training Oregon (PMTO) for parents with foster children (aged 4-12) with severe externalizing behavior problems in long-term foster care arrangements. The aim of PMTO, a relative long and intensive (6-9months, with weekly sessions) parent management training is to reduce children’s problem behavior through improvement of parenting practices. We specifically investigated whether PMTO is effective to reduce foster parenting stress. It was expected that PMTO would reduce parenting stress in foster parents, improve the quality of parenting practices, and reduce children’s problem behavior. Multi-informant (foster mothers, foster fathers, and teachers) data were used from 86 families. Multilevel analyses based on the intention to treat principle (retention rate 73%) showed that PMTO, compared to care as usual, had no significant direct, nor indirect or sleeper effects at follow-up on parenting stress, parenting behavior and child behavior problems. Earlier reported immediate effects of PMTO on reduced parenting stress at posttest disappeared at follow-up. Additional analyses on the role of non-specific intervention factors in PMTO effects showed that higher therapist fidelity scores resulted in stronger effects of PMTO on parenting responsiveness, parental explaining and autonomy granting. Unexpectedly, higher fidelity scores also predicted less decrease of parenting stress at follow-up.