In daily clinical practice, there is a growing number of patients receiving new biological agents used in the treatment of malignancies. Ipilimumab is a fully humanized monoclonal antibody approved for patients with melanoma. It acts as an immune checkpoint inhibitor, binding and blocking cytotoxic T-lymphocyte antigen-4 in order to increase the antitumor immune response. There are several reports of autoimmune responses after its use. A 74-year-old man developed a mild rash and pruritus a few hours after the second infusion of ipilimumab and 24 h after the third dose of ipilimumab, he presented with shortness of breath, proximal limb muscle weakness, and diplopia. Repetitive nerve stimulation was consistent with a postsynaptic neuromuscular junction disorder. He began therapy with corticosteroids and pyridostigmine and ipilimumab was discontinued. Following ipilimumab suspension, the patient started to improve gradually. Here, we describe a rare case of myasthenia gravis presumably related with ipilimumab's therapy. A better knowledge of these agents is necessary, in order to identify characteristics or biomarkers that may be associated with the development of potentially serious autoimmune responses.