There is a strong association between neoplasms and autoimmune diseases. Numerous autoimmune phenomena have been reported in malignancies and conversely: malignant tumors are diagnosed in increasing frequency in autoimmune conditions. We review the most common autoimmune diseases and autoantibodies found in malignancies, discuss the therapeutic role of these autoantibodies in cancer, and summarize the current knowledge on malignant transformation in autoimmunity.