OBJECTIVES: To compare the effect of treatment with zidovudine (ZDV) or a combination of ZDV and interferon-alpha (IFN-alpha) on patient Quality-of-Life (QoL); and to document changes over time in QoL. DESIGN: This study is part of a longitudinal, randomized, controlled clinical trial comparing the efficacy and tolerance of ZDV monotherapy and ZDV plus IFN-alpha. Patients were followed-up for 1 year. SETTING: Seven academic or general medical hospitals. PARTICIPANTS: Thirty-six symptomatic HIV-infected patients (Centers for Disease Control and Prevention stage IV) with a CD4+ count > or = 150 x 10(6)/l and Karnofsky Performance Status score > or = 60, who had not received ZDV or IFN-alpha before. METHODS: QoL was assessed using two self-report questionnaires (the European Organization for Research and Treatment of Cancer Core Quality-of-Life Questionnaire and an AIDS-specific questionnaire), administered before and every 3 months after the start of the treatment. RESULTS: There were no significant differences in QoL between the two treatment groups over a 1-year period. Emotional, cognitive and social functioning improved in the entire group, and patients reported fewer symptoms (for example, shortness of breath, nausea and vomiting, influenza-like symptoms, diarrhoea, lack of appetite, taste disturbances, dizziness, weight loss, trouble in vision) and a better overall QoL until month 9. Thereafter, emotional, cognitive and social functioning and overall QoL deteriorated and patients reported more symptoms. Major complaints at baseline and follow-up were associated with fatigue and emotional functioning: patients reported a strong need to rest, and felt very tired, worried, tense and irritable. CONCLUSIONS: These results show that both treatments may have a temporary beneficial effect on patient QoL. QoL research may contribute to evaluation of clinical trials and provide patients with information about the effects of treatment on their QoL, thus enabling them to make informed decision