Background <p>Several family factors are linked to later severe disruptive behavior problems, and family treatment is often recommended for treatment of adolescent disruptive behavior. Functional Family Therapy (FFT) is an evidence-based intervention designed to reduce adolescent disruptive behavior by improving family functioning. While prior research has evaluated the effectiveness of FFT in enhancing family functioning, there is limited evidence on its impact in service settings where treatment referral does not require prior involvement with the justice system.</p> Methods <p>This parallel group, open label randomized controlled trial examined the short- and long-term effectiveness of FFT on family functioning outcomes. One hundred and sixty-one youths (<i>M</i>_<i>age</i> = 14.7, 45.9% female), referred to FFT in four Norwegian Child Welfare Service organizations for disruptive behavior, were randomly assigned to FFT (<i>n</i> = 88) or treatment as usual (TAU; <i>n</i> = 73). The outcomes were parent- and youth-reported cohesion and conflict in the family; parent- and youth-reported positive involvement and positive parenting, parent-reported conflict resolution strategies (negotiation and psychological aggression); and youth-reported social support from parents and attachment (trust, communication, and alienation). Data were collected at fixed timepoints before treatment (pretest), six months after pretest (posttest), and 18 months after pretest (follow-up). A latent change score modeling approach was employed to assess differential change following treatment.</p> Results <p>Results showed no intervention effect of FFT compared to TAU on any outcome. Improvements were found between pretest and posttest for the entire sample, regardless of treatment group, on parent-reported cohesion (<i>d</i> = −0.27) and conflict (<i>d</i> = 0.30); and youth-reported social support (<i>d</i> = −0.33), cohesion (<i>d</i> = −0.23), conflict (<i>d</i> = 0.25<i>)</i>, and alienation (<i>d</i> = 0.33). Between posttest and follow-up significant improvements were found on youth-reported cohesion (<i>d</i> = 0.29) and conflict (<i>d</i> = −0.32).</p> Conclusion <p>Findings did not support the hypothesized superiority of FFT over TAU in the Norwegian context.</p> <p>ISRCTN trial registry number and date of registration (retrospectively registered): ISRCTN58861782, May 24th 2013.</p>