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Comorbid psychological distress is commonly related to chronic pain, but addressing heterogeneous comorbidities in traditional clinical settings can be difficult. Delivering individually tailored treatment using the internet could be a viable alternative. The overarching aims of the three studies in the present thesis was to investigate both treatment effects and processes in a therapist-guided and individually tailored internet-based CBT (iCBT) for individuals with chronic pain and phycological distress recruited from specialist care. Participants were recruited from a pain clinic and randomized to either iCBT or a waiting list. The participants (n = 187) received individually tailored treatments that included 6–13 modules targeting different types of psychological distress.

Study I investigated whether the guided, individually tailored iCBT could improve mood and reduce disability in individuals suffering from chronic pain and comorbid psychological distress. Intention-to-treat analyses showed larger improvements in depression, disability, pain acceptance, catastrophizing, and quality of life in the iCBT-group compared to the control condition. Between-group effect sizes were very small or small at post for the primary outcomes depression (d = 0.18) and pain interference (d = 0.22).

Study II investigated predictor variables for the iCBT participants’ (n = 95) adherence to the treatment, as well as the relationship between adherence and outcome. Results showed that treatment adherence was predicted by higher treatment credibility at baseline. Furthermore, participants who were behind schedule in the second week of the program tended to show lower adherence during the remainder of the treatment. Finally, all adherence variables predicted improvements in the outcome pain interference.

Study III describes the characteristics of text-based therapist-participant interactions during the course of the treatment and examines whether the nature of these interactions could predict adherence and outcome. Both therapist (n = 1240) and participant messages (n = 609) were categorized using a coding scheme developed from previous research. Proportions of observed behaviors were then correlated with each other, and with measures of treatment outcome and adherence. Analyses showed numerous correlations both between and within therapist and participant behaviors. No significant relationship was found between the coded behaviors and outcome; however, several significant correlations with treatment adherence were observed.