The authors report their observations gathered by follow-up studies of many years duration on 67 patients suffering from mixed connective tissue disease. The period preceding the overt disease, its duration and its characteristic symptoms were examined. Concerning the severity of the clinical course, the authors distinguished between a less and a more severe form of the disease. Organ specific manifestations occurring in the two types of the disorder, the number of active phases per year, therapeutic tools and the prognosis were analysed. The authors assume that the more severe form of the disease may have some traits resembling those of systemic lupus erythematosus, but in the meantime, in the less severe type of the disease some features shared by rheumatic disorders can be found.