The hypothesis of the research was that the serum IL-6 level and IL-6 expression in the prostate tissue are higher in patients with prostate cancer in comparison to patients with prostatic intraepithelial neoplasia (PIN), benign prostatic hyperplasia (BPH) and prostatitis . The research was based on the analysis of serum IL-6 levels and IL-6 expression in tissue samples obtained from prostate biopsy from 148 patients (82 patients with prostate cancer, 25 patients with BPH, 24 patients with PIN and 17 patients with prostatitis). Serum IL-6 levels were measured by electrochemiluminescence immunoassay (ECLIA) and IL-6 expression in the prostate tissue samples was determined by mouse anti human IL-6 antibodies (NCL-L-IL-6). Serum IL-6 levels were significantly higher in patients with poorly differentiated prostate cancer in comparison to patients with moderately differentiated prostate cancer (p = 0.007). Also, high serum IL-6 levels were found in patients with bone metastases. Serum IL-6 levels were associated with Gleason score (GS), but not with extraprostatic extension. Immunohistochemical (IHC) staining identified IL-6 expression in cytoplasm of the normal epithelial cells, premalignant and malignant prostate cells and in stromal prostate cells (mononuclear, fibroflasts and endothelial cells). The highest IL-6 expression in normal secretory cells was found in prostate tissue samples with BPH (60% of the samples had IL-6 expression in more than 50% normal secretory cells), IL-6 expression decreased in PIN and prostatitis. The lowest IL-6 expression in normal secretory cells was found in the tissue samples with prostate cancer. Moreover, basal cells had the highest IL-6 expression in the tissue samples with BPH (52% of the BPH tissue samples had in more than 50% basal cells IL-6 expression) whereas the lowest IL-6 expression in basal cells was found in the tissue samples with prostate cancer. IL-6 expression was higher in epithelial cells than in stromal cells. Epithelial malignant cells showed higher IL-6 expression than normal epithelial cells. There was no significant difference in IL-6 expression in premalignant and malignant epithelial cells (p = 0.814). Proinflammatory cytokine IL-6 has an important role in the development and progression of prostate adenocarcinoma and could be an effective target for immunotherapy.