The present invention relates to methods of treating multidrug resistance in cancerous cells with phosphodiesterase (PDE) inhibitors, e.g., PDE5 inhibitors. More specifically, the invention relates to methods of treating multidrug resistance that arises, e.g., during administration of chemotherapeutic / antineoplastic (anticancer) agents for treatment of cancer, with a PDE5 inhibitor (e.g., sildenafil, vardenafil, and tadalafil). The invention also relates to methods of treating cancer, e.g., multidrug resistant cancer, using a PDE5 inhibitor in combination with an antineoplastic therapeutic agent. Further, the invention relates to pharmaceutical compositions for treating multidrug resistant cancers comprising a PDE5 inhibitor, or a combination of a PDE5 inhibitor and an antineoplastic agent.