There are few effective treatment options available for patients with advanced melanoma. An oncolytic herpes simplex virus type 1 encoding granulocyte macrophage colony-stimulating factor (GM-CSF; Oncovex(GM-CSF)) for direct injection into accessible melanoma lesions resulted in a 28% objective response rate in a Phase II clinical trial. Responding patients demonstrated regression of both injected and noninjected lesions highlighting the dual mechanism of action of Oncovex(GM-CSF) that includes both a direct oncolytic effect in injected tumors and a secondary immune-mediated anti-tumor effect on noninjected tumors. Based on these preliminary results a prospective, randomized Phase III clinical trial in patients with unresectable Stage IIIb or c and Stage IV melanoma has been initiated. The rationale, study design, end points and future development of the Oncovex(GM-CSF) Pivotal Trial in Melanoma (OPTIM) trial are discussed in this article.