Associating with a negative peer group is related to involvement in problem behavior, yet very little research has considered the role of negative peer group affiliations in the context of "real-world" community-based treatments for problem behavior. This study examined the effects of negative peer involvement on case closure status and treatment characteristics in a large sample (N = 1,341) of adolescents (M age = 15.3 years, SD = 1.5 years) enrolled in Multisystemic Therapy services. Data were drawn from the clinical records of a nonprofit youth and family services provider. Findings suggest that negative peer involvement is significantly related to treatment failure, particularly when negative peer involvement is comprised of gang affiliation. (Contains 4 tables.)