Purpose: We investigated whether inhibition of interleukin 6 (IL-6) has therapeutic activity in ovarian cancer via abrogation of a tumor-promoting cytokine network. Experimental Design: We combined preclinical and in silico experiments with a phase 2 clinical trial of the anti-IL-6 antibody siltuximab in patients with platinum-resistant ovarian cancer. Results: Automated immunohistochemistry on tissue microarrays from 221 ovarian cancer cases showed that intensity of IL-6 staining in malignant cells significantly associated with poor prognosis. Treatment of ovarian cancer cells with siltuximab reduced constitutive cytokine and chemokine production and also inhibited IL-6 signaling, tumor growth, the tumor-associated macrophage infiltrate and angio-genesis in IL-6–producing intraperitoneal ovarian cancer xenografts. In the clinical trial, the primary endpoint was response rate as assessed by combined RECIST and CA125 criteria. One patient of eighteen evaluable had a partial response, while seven others had periods of disease stabilization. In patients treated for 6 months, there was a significant decline in plasma levels of IL-6–regulated CCL2, CXCL12, and VEGF. Gene expression levels of factors that were reduced by siltuximab treatment in the patients significantly correlated with high IL-6 pathway gene expression and macrophage markers in microarray analyses of ovarian cancer biopsies.