A 47-year-old female patient with rheumatoid arthritis (RA) treated with methotrexate, was diagnosed with metastatic melanoma. After surgical removal of the tumor, the patient start-ed treatment with ipilimumab while methotrexate was stopped. One week after initiation of ipilimumab, the patient developed typical symptoms of RA. Analgetic therapy was started. After 4 cycles of ipilimumab, melanoma progression was radiologically evident. The treat-ment plan was changed to pembrolizumab (anti-PD1 antibody), and the patient did not show active signs of RA anymore. Despite treatment with pembrolizumab, the patient died 4 months later due to tumor progression. The exact mechanism by which ipilimumab (anti-CTLA-4 antibody) provoked RA symptoms is still not fully understood. This subject needs more investigation, especially in an era in which immunotherapies are a standard therapy for patients with malignancy.