BackgroundResearch suggests that individuals with obsessive-compulsive disorder (OCD) frequently experience insomnia. Some previous studies have suggested that insomnia may predict treatment outcomes, but the evidence is limited, especially for adolescents. This study examined the prevalence of insomnia in an adolescent OCD patient sample, explored the correlation between OCD and insomnia, and tested whether levels of insomnia at baseline predict outcomes for adolescent patients receiving the Bergen 4-Day Treatment (B4DT) for OCD.MethodsForty-three adolescent OCD patients who received B4DT were selected for this study. Treatment outcome was quantified as change in Children Yale-Brown Obsessive Compulsive Scale (CY-BOCS) scores across time from pre- to posttreatment and 3-month follow-up. Insomnia symptoms were measured by the Bergen Insomnia Scale (BIS). Linear mixed models were used to examine the relationship between the BIS and changes in CY-BOCS scores. We controlled for symptoms of general anxiety disorder measured by the GAD-7 and depression symptoms measured by the PHQ-9.ResultsIn this sample, 68.4% of the patients scored above the cutoff for insomnia on the BIS. There was a moderate correlation between baseline CY-BOCS and BIS that did not reach statistical significance (r = .32, p = .051). High BIS scores before treatment were significantly associated with poorer treatment outcomes, as measured by changes in CY-BOCS over time (p = .002). The association between baseline insomnia and change in OCD symptoms remained significant (p = .033) while controlling for GAD-7 and PHQ-9.ConclusionInsomnia is common among adolescents with OCD, and these data suggest that these patients may be at increased risk for poor treatment outcomes. Future research to explore mechanisms and adjunctive treatments is warranted.Trial registrationThe study was approved by the Regional Committee for Medical and Health Research Ethics of Northern Norway (REK Nord: 2023/606482).