Background: Sleeping difficulties are an increasing problem for youths, but there is a lack of treatment research for this age group. Aim: The aim of this study was to investigate the effects of Cognitive Therapy for Insomnia (CT-I) on youths with primary insomnia; this treatment has never been tested on youths before. Method: The study was conducted according to an AB single-case experimental phase design, with pre-tests and post-tests. After 7â??10 days of baseline assessment, three youths aged 16â??18 (2 male) with primary insomnia participated in a 7-week long treatment. A sleep diary was used throughout the treatment. A follow-up assessment including one week with a sleep diary was conducted 3 months later. Visual inspection was used to analyze outcome. Results: Insomnia severity was greatly reduced for all participants after treatment. Daily measures showed that sleep onset latency was reduced for two participants but no change in total sleep time was confirmed. Daytime symptoms fluctuated for the participants. The insomnia-specific psychological processes were reduced to varying extents. These results were maintained at the follow-up measure. Conclusions: CT-I may be a promising treatment for youths with insomnia and the treatment should be tested further in randomized controlled studies.