BackgroundSleep disturbances, including insomnia, are common in adult Attention Deficit Hyperactivity Disorder (ADHD). Treatment of choice for insomnia is cognitive behavioral therapy (CBT-i), but evidence is lacking for CBT-i in patients with ADHD.The purpose of this study was to investigate if patients with insomnia and other sleep problems, at a specialist clinic for ADHD, benefit from a group delivered behavioral treatment based on CBT-i; whether insomnia severity improves following this treatment.MethodsThis pragmatic within-group pilot study with a pre to post and three-month follow-up design was set at a specialist psychiatric out-patient clinic for adult ADHD.As an adjunct to care-as-usual at the clinic, a CBT-i-based group treatment targeting several sleep problems prevalent in the ADHD–population, was offered as 10 weekly 90-min group sessions and scheduled telephone support.All outcome measures were subjectively reported by participants. Data analyzed with dependent t-tests according to intent-to-treat.ResultsNineteen patients (37 [SD 13.7] years; 68% female) with ADHD and subjectively reported sleep problems provided informed consent and pre-treatment measures. Patients had suffered from sleep problems for 15.3 [SD 13.4] years, 42% used sleep medications, 79% used stimulant medication(s).At post-treatment, insomnia severity (Insomnia Severity Index; score range 0–28) had improved with 4.5 points (95% CI, 2.06–6.99, p = .002), at 3 months with 6.8 points (95% CI, 4.71–8.91, p < .0001) from pre-treatment.ConclusionsCBT-i adjusted for ADHD is promising for improving insomnia severity in adult patients at specialist psychiatric out-patient clinics, who suffer from ADHD and sleep disturbances.Trial registrationStudy registered with the Regional ethical review board in Stockholm, January 13th 2016, Study id: 2015/2078–31/1. Study registered retrospectively with Clinicaltrials.org, February 21st 2019, ID: NCT03852966.