Ethnic minority families in Europe are underrepresented in mental health care-a profound problem for clinicians and policymakers. One reason for their underrepresentation seems that, on average, ethnic minority families tend to perceive externalizing and internalizing child behavior as less problematic. There is concern that this difference in problem perception might limit intervention effectiveness. We tested the extent to which ethnic differences in problem perception exist when ethnic minority families engage in mental health service and whether lower levels of problem perception diminish parenting intervention effects to reduce disruptive child behavior. Our sample included 136 mothers of 3- to 8-year-olds (35% female) from the 3 largest ethnic groups in the Netherlands (43% Dutch; 35% Moroccan; 22% Turkish). Mothers reported on their child’s externalizing and internalizing behavior and their perception of this behavior as problematic. They were then randomly assigned to the Incredible Years parenting intervention or a wait list control condition. We contrasted maternal reports of problem perception to teacher reports of the same children. Moroccan and Turkish mothers, compared with Dutch mothers, perceived similar levels of child behavior problems as less problematic, and as causing less impairment and burden. Teacher problem perception did not vary across children from different ethnic groups. Importantly, maternal problem perception did not affect parenting intervention effectiveness to reduce disruptive child behavior. Our findings suggest that ethnic differences in problem perception exist once families engage in treatment, but that lower levels of problem perception do not diminish treatment effects.