Purpose Interferon (IFN) -based adjuvant therapy in melanoma is associated with significant side effects, which necessitates evaluation of health-related quality of life (HRQOL). Our trial examined the HRQOL effects of adjuvant pegylated IFN-α-2b (PEG-IFN-α-2b) versus observation in patients with stage III melanoma. Methods A total of 1,256 patients with stage III melanoma were randomly assigned after full lymphadenectomy to receive either observation (n = 629) or PEG-IFN-α-2b (n = 627): induction 6 μg/kg/wk for 8 weeks then maintenance 3 μg/kg/wk for an intended total duration of 5 years. The European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire C30 was used to assess HRQOL. Results At 3.8 years of median follow-up, for the primary end point, recurrence-free survival (RFS), risk was reduced by 18% (hazard rate = 0.82; P = .01) in the PEG-IFN-α-2b arm compared with observation. Significant and clinically meaningful differences occurred with the PEG-IFN-α-2b treatment arm compared with the observation group, showing decreased global HRQOL at month 3 (−11.6 points; 99% CI, −8.2 to −15.0) and year 2 (−10.5 points; 99% CI, −6.6 to −14.4). Many of the other scales showed statistically significant differences between scores when comparing the two arms. From a clinical point of view, important differences were found for five scales: two functioning scales (social and role functioning) and three symptom scales (appetite loss, fatigue, and dyspnea), with the PEG-IFN-α-2b arm being most impaired. Conclusion PEG-IFN-α-2b leads to a significant and sustained improvement in RFS. There is an expected negative effect on global HRQOL and selected symptoms when patients undergo PEG-IFN-α-2b treatment.