Undifferentiated connective tissue disease (UCTD) refers to a particular connective tissue disease that is difficult to attribute to a specific type of connective tissue disease according to today’s known diagnostic (or classification) criteria and tests. Clinical manifestations The patient has the common symptoms of connective tissue disease: polyarthritis, Raynaud’s phenomenon, myositis, vasculitis, plasmacytosis, interstitial lung changes, etc., and serum immunological tests are often positive for antinuclear antibodies, rheumatoid factor or LE cells, and elevated immunoglobulins. In the case of an individual, he has common clinical symptoms of connective tissue diseases such as SLE, RA, PSS or PM/DM, but does not have specific antibodies associated with these diseases and does not meet the diagnostic criteria for his unique disease, (e.g. SLE, RA, PSS, PM/DM or MCTD). Clinical symptoms may be early manifestations of a distinct disease that can evolve into a defined connective tissue disease as the disease progresses, or maintain these symptoms for a long period of time, or reduce, improve or disappear with treatment. Therefore, the clinically undiagnosed connective tissue disease is called UCTD. Diagnosis; Before making a diagnosis of UCTD, a detailed history, a thorough physical examination and necessary laboratory thrifty investigations are necessary. In order to avoid missed diagnosis, misdiagnosis and delayed treatment. For Raynaud’s phenomenon, we should analyze whether it is an isolated symptom or a part of some connective tissue disease, and further check ANA, anti-dsDNA antibody, anti-Sd-70 antibody and anti-adhesion point antibody. 2. For patients with polyarthritis, if ANA is negative, rheumatoid factor is negative, and there are no bone changes in the joints, close follow-up should be performed. 3.For young women with hemolytic anemia or thrombocytopenia, with glomerulonephritis and central nervous system involvement, they should be highly alert for SLE and further tested for ANA, anti-dsDNA and anti-Sm antibodies. 4. For those who have thickened, thickened and tightened skin at the fingertips and swollen hand joints, they should be examined for heart, lung, intestine and kidney lesions, and examined for anti-Scl-70 and anti-adnexal antibodies, etc. Myositis is a common symptom in many connective tissue diseases. However, it is more common in PM/DM, and muscle enzyme profile and electromyography should be performed promptly, and muscle biopsy should be performed if necessary to exclude PM. In conclusion, before diagnosing UCTD, it is important to make detailed and necessary examinations to figure out whether these clinical manifestations and immunological abnormalities are partial or early manifestations of a disease. It has been suggested that MCTD should be included in the scope of UCTD because MCTD will differentiate sooner or later. We believe that MCTD has its own unique diagnostic criteria, and that differentiating it from UCTD for further follow-up is beneficial to understanding and exploring the pathogenesis of connective tissue disease. Treatment Treatment of UCTD is usually symptomatic, with a small amount of hormone or immunosuppressive therapy if necessary. Close follow-up of differentiation is more important. It is believed that with advances in clinical immunology and molecular bioimmunology, UCTD may be a subtype of a unique connective tissue disease or another novel, independent connective tissue disease.