The possibility of three or more autoimmune diseases occurring in the same patient cannot be fortuitous and suggests a pathogenic relationship between each of them. In the light of 4 personal cases, the authors have recorded 87 reports of such associations in the literature, an analysis of which leads them to propose a classification of three types of multiple autoimmune syndrome. The grouping of these syndromes under a single heading should make the research and analysis of these morbid associations easier. Moreover, the classification adopted by the authors allows a more precise definition of patients with at least two autoimmune diseases and so helps to recognize the onset of a third autoimmune disease at a later date. Multiple autoimmune syndromes can be classified in 3 groups according to the prevalence of their associations one with another. Type I comprises myasthenia, thymoma, polymyositis and giant cell myocarditis, this association having a single pathogenic mechanism. Type II includes the Sjögren's syndrome, rhumatoid arthritis, primary biliary cirrhosis, scleroderma and autoimmune thyroid disorders. Type III groups together 10 autoimmune diseases (autoimmune thyroid disease, myasthenia and/or thymoma, Sjögren's syndrome, pernicious anaemia, idiopathic thrombocytopaenic purpura, Addison's disease, insulin-dependent diabetes, vitiligo, autoimmune haemolytic anaemia, systemic lupus erythematosus) for which a genetic predisposition (phenotype HLA B8 and/or DR3 or DR5) seems to be an important factor.