Background: Internet-supported and therapist-guided cognitive behavior therapy (ICBT) is effective for a range of problems in the short run, but less is known about the long-term effects with follow-ups of two years or longer.

Methods: We reviewed studies in which the long-term effects of guided ICBT were investigated. Following literature searches in PubMed and other sources meta-analytic statistics were calculated for 14 studies involving a total of 902 participants, and an average follow-up period of three years. Studies were from Sweden (n=11) or the Netherlands (n=3).

Results: Long-term outcome studies were found for panic disorder, social anxiety disorder, generalized anxiety disorder, depression, mixed anxiety and depression, obsessive-compulsive disorder, pathological gambling, stress and chronic fatigue. The duration of the treatments was usually short (8-15 weeks). The pre-to follow-up effect size was Hedge's g = 1.52, but with a significant heterogeneity. The average symptom improvement across studies was 50%. Treatment seeking in the follow-up period was not documented and few studies mentioned negative effects.

Conclusion: While effects may be overestimated, it is likely that therapist-supported ICBT can have enduring effects. Long-term follow-up data should be collected for more conditions and new technologies like smartphone-delivered treatments.