Advanced melanoma has defied treatment advances for several decades. Immunotherapy with high-dose interleukin-2 or interferon-α has been beneficial in some cases, but significant toxicities limit its use. Cytotoxic T-lymphocyte antigen-4 (CTLA-4) signaling switches off T-cell activation and induces immune tolerance. Inhibiting CTLA-4 prolongs the antitumor T-cell response, reversing tolerance. Ipilimumab is a first-in-class anti-CTLA-4 monoclonal antibody, currently under review by the Food and Drug Administration for pretreated melanoma. Ipilimumab has shown durable responses and manageable toxicities in a large phase 3 clinical trial in patients with advanced melanoma. Variable response patterns have been observed, including: (1) response in baseline lesions; (2) a slow, steady decline in tumor burden; (3) response after an increase in tumor burden; and (4) response in index and new lesions accompanied by the appearance of other new lesions. Although responses (1) and (2) may be captured using standard methods, atypical responses (3) and (4) would be classified as progressive disease using conventional assessments. Patients on ipilimumab may have delayed responses or durable stable disease even after apparent disease progression, therefore using new immune-related response criteria is recommended to avoid premature treatment withdrawal. This review compares and contrasts responses to ipilimumab with those after chemotherapy, and discusses treatment implications.