Study of the Drug Ipilimumab for Metastatic Merkel Cell Carcinoma

Background: \- Ipilimumab is a drug used to treat melanoma that cannot be treated surgically. It targets a molecule found on T-cells in the human immune system. Blocking these molecules on the T-cells might allow the cells to help destroy melanoma cells more effectively. This drug has also been studied in other cancers such as prostate cancer and lung cancer, but not yet in Merkel cell carcinoma (MCC). Researchers think therapy like ipilimumab that enhances the immune system may be effective against MCC. They want to study how safe the drug is and its effect on the immune system and tumors. Objectives: \- To determine the number of subjects with MCC who take the study drug that remain alive 12 months later. Eligibility: \- Adults 18 years and older who have metastatic MCC. Design: * Participants will be screened with a medical history and physical exam. * Participants will receive the study drug 4 times, one dose every 21 days. After the 4 visits, participants will receive a maintenance dose of the drug every 12 weeks until the drug is no longer beneficial. * They will receive the drug through a plastic tube usually inserted in a vein on the arm. * It will take 90 minutes to give each dose. * At all visits, participants will be screened with a medical history, physical exam, and blood tests. Any tumors on their skin will be measured and photographed. * Every 12 weeks during the study and maintenance period, participants will have a CT scan. Throughout the study and maintenance period, they will have blood and skin tests.