Interleukin-6 (IL-6) is an immunomodulatory cytokine produced by both normal cells and tumor cells, including melanoma cells. The specific biological function of IL-6 in melanoma is unknown. The present study examined whether the serum concentration of IL-6 can predict prognosis in patients with metastatic melanoma. IL-6 was measured by ELISA in serum samples from 103 patients with metastatic melanoma obtained before IL-2-based immunotherapy. Patients with metastatic melanoma had higher serum IL-6 than healthy individuals (median 3.4 ng/l, range 0.3-93 ng/l vs. median 1.4 ng/l, range 0.25-22.5 ng/l, P<0.0001). Pretreatment serum IL-6 was elevated in 43% of the patients. Patients with elevated pretreatment serum IL-6 had shorter overall survival (OS) compared with patients with normal serum IL-6 (P<0.0002). The median OS was 10.8 months [95% confidence interval (CI): 8.86-13.46] in patients with normal serum IL-6 compared with 4.5 months (95% CI: 3.04-7.39) in patients with elevated serum IL-6. Multivariate Cox analysis showed that serum IL-6 [hazard ratio (HR)=1.82, 95% CI: 1.19-2.78, P=0.006] and serum lactate dehydrogenase (HR=2.02, 95% CI: 1.31-3.11, P=0.001) were independent prognostic biomarkers of OS. A combination variable of elevated serum IL-6 and elevated serum lactate dehydrogenase almost quadrupled the risk of early death (HR=3.67, 95% CI: 2.17-6.20, P<0.0001) compared with patients with normal serum levels of these two biomarkers. Elevated serum IL-6 is an independent prognostic biomarker of short OS in patients with metastatic melanoma. A larger retrospective study is ongoing to confirm the findings. To validate serum IL-6 further as a prognostic biomarker, a prospective study is required.