Connective tissue diseases (CTD) are common causes of disseminated small – focal lung damage. The complexity of diagnostic approach in such cases occur due to intolerance of many symptoms and syndromes, that can also relate to diseases of other different etiologies. It is often not possible to identify the pathognomonic syndrome in CTD, as well as to establish an accurate diagnosis after a long time. This complicates the selection of effective treatment and, accordingly, the inability to predict the course and development of the disease. Sometimes, due to the intersection of symptoms, the diagnosis sounds like «MCTD» (mixed connective tissue diseases) – a rare systemic connective tissue disease, characterized by a combination of individual signs of systemic lupus erythematosus, systemic scleroderma, rheumatoid arthritis, polymyositis. This publication presents a clinical case of a long-term course of mixed MCTD in a patient who has been observed for more than 10 years in different hospitals, and by outpatient observation, and within this year’s differential diagnosis with other CTD and infectious diseases was held, including rheumatic fever.