Merkel cell carcinoma (MCC) is a rare skin cancer that occurs more frequently after organ transplantation or B-cell malignancy, conditions of suppressed or disordered immunity. To assess further whether immune suppression increases MCC risk, we studied its occurrence in a cohort of 309365 individuals with acquired immunodeficiency syndrome (AIDS) by using linked AIDS and cancer registries. We identified six cases of MCC, corresponding to a relative risk of 13.4 (95% CI 4.9-29.1) compared with the general population. These results suggest that immune suppression induced by the human immunodeficiency virus increases MCC risk.