Presented study was conducted to investigate the prognostic significance of the coexpression of serum interleukin-6 (IL-6) and tumor necrosis factor-a (TNF-a) in breast cancer, by correlating their presence with clinicopathological characteristics indicative of tumor progression and the overall survival of breast cancer patients. One hundred twelve consecutive patients with primary breast cancer were prospectively included and evaluated. Serum concentrations of IL-6 and TNF-a were measured by quantitative sandwich enzyme immunoassay (ELISA). Median split was used to subdivide patients with low or high IL-6 and TNF-a levels. A positive association between the expression of the two cytokines was found. The coexpression of high IL-6 and TNF-α was independently associated with extended lymph node (>3) involvement (aOR, 7.8) and lymphovascular invasion (aOR, 14.1), increasing the prognostic significance of each cytokine separately; it also provided additional prognostic information regarding survival, defining a high-risk subgroup of patients with significantly shorter survival and higher risk of death compared to patients with both cytokines low (aHR, 4.45) and patients with only one cytokine high (aHR, 3.63). Our findings suggest that the coexpression of these two cytokines could be used clinically as a useful tumor marker for the extension and the outcome of the disease.