Disruptive behavior during adolescence is linked to severe problems for the youths later in life and poses challenges to the families, schools, and treatment systems these youths meet. This randomized controlled trial was conducted to examine the short- and long-term effectiveness of functional family therapy (FFT) for adolescents aged 11-17 referred for disruptive behavior to Child Welfare Services in Norway. One hundred sixty-one youths (Mage = 14.7, 45.9% female) were randomly assigned to FFT (n = 88) or treatment as usual (TAU, n = 73). Primary outcomes were parent- and teacher-reported youth aggressive behavior, rule-breaking behavior, internalizing problems, and social skills; youth self-reported delinquency (SRD) and negative peer involvement; and teacher-reported academic performance and adaptive functioning. Outcomes were collected before treatment (pretest), 6 months after pretest (posttest), and 18 months after pretest (follow-up). The results showed no intervention effect for FFT compared to TAU between pretest and posttest (p > .05). Significant improvements between pretest and posttest were found for youth receiving both FFT and TAU on parent-reported aggressive and rule-breaking behavior, internalizing problems, and social skills (ranging from d = 0.56 to -0.45) and youth SRD (d = 0.29). Between posttest and follow-up, however, a significant intervention effect in favor of TAU was found for parent-reported youth internalizing (d = 0.27). Significant improvements between posttest and follow-up were also found for youth receiving both FFT and TAU on parent- and teacher-reported aggressive behavior. Findings did not support the hypothesized superiority of FFT over TAU. (PsycInfo Database Record (c) 2024 APA, all rights reserved).