To characterize the clinical features of mixed connective tissue disease (MCTD) in childhood, 23 reported cases in Japan were analyzed for their symptoms and signs, and laboratory data. The earliest onset of the disease was found in 5 years of age, and the number of patients were increased with age. Over 80% of patients was girls. Raynaud's phenomenon preceded in most patients to the increased disease activity manifested by spiky fever, arthritis, skin rashes, and myositis. The characteristic laboratory findings were demonstrated by anti-nuclear antibody (speckled type), anti-RNP antibody, rheumatoid factor, and marked hyper-gammaglobulinemia. The overall prognosis was fairly good. But cardiac involvement was the most serious problem in the early stage of the disease, and a very few children was accompanied with renal disorders. The central nervous system was rarely involved. As the long term follow-up of the children was not accomplished yet, pulmonary fibrosis or pulmonary hypertension was left for the further documentation.