Mixed connective tissue disease deserves to be a distinct disease entity due to the persistent citation of this disease in the literature since the original description by Sharp in 1972, in spite of the presence of several criticisms against the independency of this disease. The characteristic features of mixed connective tissue disease are: 1) the presence of anti-U1snRNP antibody with high titers in sera, 2) an increased frequency of HLA-DR4 in the leukocytes, and 3) death due to pulmonary hypertension.