The aim of this study was to compare the quality-of-life (QOL) effects of interleukin-2 (IL-2) alone with those of IL-2 plus histamine dihydrochloride in the setting of a multicenter, randomized trial for patients with metastatic melanoma. QOL data were collected from July 1997 to March 2000 during a phase III trial comparing subcutaneous histamine plus IL-2 and IL-2 alone. Prior to each treatment cycle, patients completed the 76-item Quality-of-Well-Being Scale-Self-Administered (QWB-SA) questionnaire, the Overall State of Health (OSH) item, and the General Health Perception (GHP) item. A longitudinal data analysis using the generalized estimating equations approach was performed to compare changes in QWB-SA scores over time between treatment groups, and predicted QWB-SA scores from the regression analysis were used to calculate quality-adjusted survival duration over the 12-month study period. QOL analyses were conducted for all randomized patients (intent-to-treat overall population, ITT-OA) and all patients who had liver metastases at randomization (ITT-LM population). In the ITT-OA population, differences in QWB-SA scores over time between the histamine plus IL-2 group (150 patients) and the IL-2 alone group (151 patients) were not significant (P=0.511, type III F test). In the ITT-LM population (53 histamine plus IL-2 patients and 73 IL-2 alone patients), changes in QWB-SA scores over time favored the histamine plus IL-2 group (P=0.018, type III F-test). In both the ITT-OA and ITT-LM populations, QWB-SA scores deteriorated more quickly over time in the IL-2 alone group than in the histamine plus IL-2 group, resulting in a significant difference in median quality-adjusted survival duration in favor of the histamine plus IL-2 group by 31.3 days in the ITT-OA population (P=0.007, Mann-Whitney U-test), and 50.2 days in the ITT-LM population (P=0.011). OSH and GHP scores did not differ between groups. The addition of subcutaneous histamine dihydrochloride to IL-2 treatment improved median quality-adjusted survival duration and did not adversely affect QOL in patients with malignant melanoma.