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Undifferentiated connective tissue diseases are an actually existing clinical disease state of unknown origin. They clinically resemble autoimmune diseases with the presence of a kind of autoantibody, which does not meet the diagnostic criteria for any defined connective tissue disease. Evolution is the most prominent feature of undifferentiated connective tissue diseases, of which, 30% could evolve into connective tissue diseases, 70% remain in the initial state. The severity of connective tissue diseases evolving from undifferentiated connective tissue diseases tends to be mild with low-grade major organ injury. Clinical manifestations and evolution process reveal that undifferentiated connective tissue diseases are an important member in the spectrum of autoimmune diseases.