Objective: To assess feasibility and explore preliminary effectiveness of an online cognitive-behavioral group intervention (Op Koers Online) to prevent and/or reduce psychosocial problems by teaching use of active coping skills to adolescents (ages 12 to 18) with chronic illness. Method: Adolescents who signed up for the chat intervention were asked to complete online questionnaires at baseline and postintervention (after 8 weeks). Feasibility was evaluated based on attendance (missed sessions, dropout rate and homework completion), technological issues and with an evaluation questionnaire. Preliminary effectiveness was evaluated with standardized questionnaires: Op Koers Online Questionnaire (disease-related coping skills), Youth Self- Report (emotional and behavioral functioning), Pediatric Quality of Life Inventory (Health-Related Quality of Life [HRQoL]). Mean scale scores postintervention were compared with baseline with paired-samples t tests. Effect sizes were calculated. Results: In total, 33 adolescents participated in the intervention, 29 adolescents completed the questionnaires at baseline. Regarding postintervention questionnaires, 25 adolescents completed the evaluation questionnaires and 23 adolescents completed all questionnaires postintervention. Dropout rate was 6%. In 1 session (2%), there were technological issues that caused the session to stop. Participants' overall satisfaction was high. Regarding effectiveness, participants improved significantly in the use of total coping skills and the coping skills "information seeking and giving" and "social competence" after the intervention compared with baseline. Participants also reported significantly fewer withdrawn/depressed behavior and scored significantly better on emotional and psychosocial HRQoL after following the intervention. Conclusions: This pilot study indicates that Op Koers Online is feasible and potentially effective. Further research (a randomized controlled trial) is needed to establish the effects of the intervention.