Primary insomnia, as defined by DSM-IV-TR, refers to a persistent sleep disturbance which is not connected to a current psychiatric or physical condition, but significantly impairs social and occupational functioning. This study explored the impact of Cognitive Behavioral Therapy for Insomnia (CBT-i) on sleep, daytime functioning and health-related quality of life (HRQoL). Next, we investigated which factors predicted positive treatment outcome by examining demographics, insomnia characteristics, baseline levels of daytime function, HRQoL, sleep-disruptive beliefs and psychological health on post-treatment sleep quality, daytime function and HRQoL. 138 consecutive primary insomnia patients completed questionnaires pre- and post-treatment and at 6 months follow-up. After CBT-i, robust clinical improvements were observed in sleep, daytime function and HRQoL, regardless of age, gender, type or duration of the complaint. Patients with pre-treatment severe insomnia, pronounced daytime impairment and low psychological well-being benefited most.