The processes that vary in nature and underlie the pathogenesis of malignant neoplasms (MNs) and immune-mediated inflammatory rheumatic diseases (RDs) share just the same some common features. Thus, many early-stage neoplasias can mask autoimmune diseases, requiring that physicians of various specialties should comply with the principle of cancer alertness. When specific cancer treatments (immunotherapy, cytostatic therapy, radiotherapy) are performed, there may be certain rheumatic syndromes (arthritis, myalgia, serositis, etc.) that require a differential diagnosis. At the same time, the course of a number of RDs (rheumatoid arthritis, Sjögren's syndrome, and polymyositis) can be accompanied by the development of MNs, which is relevant for real clinical practice and calls for further investigation.The community of etiological factors and a genetic predisposition in the development of RDs and MNs remain to be of no less importance. At the same time, therapists should pay attention to the presence of rheumatic masks in many MNs, which undoubtedly prolongs the time between onset of the first symptoms and correct diagnosis.