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-alpha-2b (PEG-IFN-alpha-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-alpha-2b (n = 627): induction 6 micrograms/kg/wk [DOSAGE ERROR CORRECTED] for 8 weeks then maintenance 3 micrograms/kg/wk [DOSAGE ERROR CORRECTED] 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-alpha-2b arm compared with observation. Significant and clinically meaningful differences occurred with the PEG-IFN-alpha-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-alpha-2b arm being most impaired. CONCLUSION: PEG-IFN-alpha-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-alpha-2b treatment.