The present study was a RCT comparing 4 conditions: 1) Internet-based intervention ‘Featback’, 2) online support from an expert patient, 3) Featback supplemented with online support from an expert patient, and 4) a waiting list control condition. This study had two aims: 1) To investigate the effectiveness and cost-effectiveness of 8 weeks of Featback, in comparison to Featback with online support from an expert patient, online support from an expert patient without Featback, and a waiting list control condition. In total, 355 participants enrolled in the study, 43% of which indicated never to have received treatment for their eating disorder, while the average duration of the eating disorder across the sample was more than 10 years. Study dropout rates were acceptable, with the respective number of participants completing assessments at baseline, post intervention, and 3, 6, 9 and 12 month follow up being 355 (100%; T0), 280 (79%; T1), 252 (71%; T2), 244 (69%; T3), 233 (66%; T4) en 242 (68%; T5). Featback only, Featback plus online expert-patient support and online expert-patient support only were effective in reducing eating disorder symptoms after eight weeks compared to a waiting list control condition. On the long term, the difference between the participants allocated to the waiting list control condition and the other conditions disappeared, indicating no long-term differences between the conditions regarding eating disorder symptoms. No significant differences between the conditions were found regarding QALYs, health care costs and societal costs. Nonsignificant differences in QALYs were in favor of the Featback conditions and the lowest societal costs per participant were observed in the Featback only condition (€16,741) while the highest costs were seen in the care-as-usual condition (€28,479). The Featback only condition had the highest probability of being efficient compared to the alternatives for all acceptable willingness-to-pay values.