This study examined research attrition in clinical service settings by comparing psychotherapy research completers and dropouts in a private therapy practice. Seventy-seven children 7–12 years old enrolled in the Resilience Builder Program® (RBP), a manualized group therapy created and administered in a private practice. Children had social impairments, and most were diagnosed with Attention Deficit Hyperactivity Disorder (ADHD) and/or anxiety disorders. Results found that compared to completers, research dropouts had significantly greater social deficits, disruptive behavior problems, affective problems, medication use, and were more likely to be ethnic minorities. We discuss implications for research recruitment and retention in clinical service settings.