Disadvantaged family socioeconomic status (SES) is often assumed to diminish parent training program effectiveness. In examining effects of SES, influences of initial problem severity have been largely ignored. In the present meta-analysis, we examined (a) whether there is a differential influence of SES on parent training effectiveness at immediate posttreatment and at 1-year follow-up-controlling for levels of initial problem severity--and (b) whether SES interacts with initial problem severity in its effect on program effectiveness. Seventy-five studies on parent training program effectiveness to reduce disruptive child behavior were included. Separate analyses were conducted for immediate posttreatment and approximately 1-year follow-up assessments. Immediately posttreatment, disadvantaged samples benefited less from parent training, but only when they had low levels of initial problem severity. At follow-up, disadvantaged samples benefited less from parent training regardless of initial problem severity. Initial problem severity was a strong predictor of effect sizes both immediately posttreatment and at follow-up. Parent training programs are equally effective for disadvantaged and nondisadvantaged families immediately posttreatment, at least when initial problems are severe. Maintenance of treatment gain, however, seems harder for disadvantaged families, suggesting that more sustained family support may be needed.