Myeloid-derived suppressor cells (MDSC) play a significant role in tumor-induced immune suppression. Targeting their function could improve antitumor therapies. Previously, we demonstrated that phosphodiesterase 5 (PDE5) inhibition in MDSCs augmented antitumor immunity in murine models. Here, we show how the addition of the PDE5 inhibitor, tadalafil, in a patient with end-stage relapsed/refractory multiple myeloma reduced MDSC function and generated a dramatic and durable antimyeloma immune and clinical response. Strategies targeting MDSC function with PDE5 inhibitors represent a novel approach that can augment the efficacy of tumor-directed therapies. Cancer Immunol Res; 2(8); 725–31. ©2014 AACR.