The promising new class of immunomodulating antibodies directed against cytotoxic T-lymphocyte antigen-4 (CTLA-4) has been extensively tested in clinical trials and found to be active against cutaneous melanoma and other tumor histotypes. Inhibition of CTLA-4 characteristically induces well-identified side effects for which the definition "immune-related adverse events" (irAEs) has been proposed. IrAEs mainly include colitis/diarrhea, dermatitis, hepatitis, and endocrinopathies; uveitis, nephritis, and inflammatory myopathy also have been reported occasionally. These unique side effects are likely a direct result of breaking immune tolerance upon CTLA-4 blockade and are generally mild, reversible, and manageable, following specific treatment guidelines that include symptomatic therapies or systemic corticosteroids. However, patient-physician communication and early treatment are also emerging as critical issues to successfully manage irAEs, thus avoiding major complications. The major experience in identifying and managing CTLA-4 treatment-related side effects has derived from studies in melanoma patients; nevertheless, accumulating clinical experiences are clearly demonstrating that irAEs are class-specific events, and that they are fully overlapping in patients with tumors of different histotypes. This review provides an overview of current safety data on CTLA-4 antagonists and of available strategies to optimize their clinical use in cancer patients.