The present study was undertaken to evaluate the prognostic significance of the serum levels of interleukin 6 (IL-6) in patients with prostate cancer. Serum IL-6 levels were measured in 74 patients with prostate cancer. The tumor was stage B in 23 patients, stage C in 14 patients, and stage D in 37 patients. Prognostic significance of tumor histology, performance status (PS), bone metastasis, serum prostate-specific antigen (PSA) level, serum alkaline phosphatase (ALP) level, serum lactate dehydrogenase level, serum IL-6 levels, and hemoglobin on disease-specific survival was assessed using univariate and multivariate Cox's proportional hazards model analyses. Serum IL-6 was significantly correlated with the clinical stage of prostate cancer. Univariate analysis of all patients demonstrated that an extent of disease (EOD) on bone scanning > or = 1, IL-6 > or = 7 pg/ml, PS > or = 1, PSA > 100 ng/ml, and ALP > 620 IU/liter were associated with a significantly lower survival rate than their respective counterparts. In multivariate analysis, however, the only two significant prognostic factors were EOD and IL-6. In 51 patients with stage C and stage D prostate cancer, univariate analysis showed that EOD > or = 1, IL-6 > or = 7 pg/ml, PS > or = 1, PSA > 100 ng/ml, LDH > 200 IU/liter, and ALP > 620 IU/liter were significantly related to survival, whereas multivariate analysis again demonstrated that EOD > or = 1 and IL-6 > or = 7 pg/ml were significant prognostic factors. These results indicate that the serum IL-6 level is a significant prognostic factor for prostate cancer as well as EOD.