Patients with malignancies often present with signs of inflammatory reactions such as elevated erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP). Since interleukin-6 (IL-6) is a possible regulator of these reactions and has been proposed as a predictor of prognosis, the aim of the study was to analyse its clinical significance in patients with renal cell carcinoma. Serum samples were collected from 196 patients before any treatment. IL-6 was analysed by an enzymelinked immunoassay and compared with tumour grade, stage, acute-phase reactants and survival. Patients with renal cell carcinoma had significantly higher IL-6 levels (mean 28.1 ± 63.4 ng/l; median 8.3 ng/l) compared with controls (mean 1.7 ± 2.6 ng/l; median 0.5 ng/l; P < 0.001). Serum IL-6 levels in patients with distant metastases were significantly higher than for patients with tumours confined to the kidney ( P = 0.02). This difference was more pronounced when serum IL-6 levels in patients with poorly differentiated tumours were compared with well-differentiated tumours ( P < 0.001). A significant correlation between the acute-phase reactants CRP, ESR and IL-6 levels was found. Survival time was significantly shorter ( P = 0.001) for patients with IL-6 levels above the median serum level compared with patients with lower levels. Similar significant prognostic results were obtained in the group of patients with metastatic disease, but not in group of patients with stage I–III. Serum levels of IL-6 correlated to tumour stage, grade and acute-phase reactants. Increased levels were related to the presence of metastases and adverse survival. Serum IL-6 proved univariate prognostic information but this prognostic significance was lost using a multivariate analysis.