Bouts of excessive warty are common across the lifespan, increasing in frequency and complexity during adolescence and adulthood, and are found in several psychiatric disorders, particularly the anxiety disorders. There are evidence-based treatments for adolescents with anxiety disorders but psychological treatments designed specifically to target excessive worrying in adolescents are rare. Intolerance of uncertainty (lU), a cognitive predisposition described as a fear of the unknown, is highly associated with worry among adolescents. This study investigated the feasibility, acceptability, and preliminary efficacy of IU-focused cognitive behavioral therapy (IU-CBT) for adolescents with excessive worry. Twelve participants (aged 13-17 years) with excessive worry, irrespective of psychiatric diagnosis, were provided weekly face-to-face sessions, primarily including therapist and self-guided exposure to situations involving uncertainty. Sessions were supplemented with an internet-delivered educational program for parents, designed to teach parents about worry, 11. 7 and helpfill parental behaviors. The main outcome measure was the Penn-State Worry Questionnaire for Children (PSWQ-C). The. treatment was well tolerated with no dropouts and families reporting being satisfied with the treatment. Participants were able to grasp the notion of lU and its relationship to worry and avoidance. Signant reductions were observed for self-reported worry, anxiety, depression, IU, and parent-reported worry, as were signant increases in global functioning. Based on a clinician rating 58.3% were categorized as much or very much improved at posttreatment, rising to 66% at 3-month follow-up. Participants with generalized anxiety disorder (GAD) benefitted more from treatment than those with social anxiety disorder. The findings suggest that this lU focused psychological intervention is acceptable and feasible to adolescents with excessive worry but may be most effective for those with GAD.