The reported prevalence and incidence of connective tissue disorders are quite variable, depending on differences in study methodology. Most important differences are the study duration, the classification criteria used for diagnosis and the country in which the study was undertaken. Sjögren's syndrome has the highest prevalence ranging between 0.5 and 3% of a given population. The prevalence of systemic lupus erythematosus (SLE) is estimated between 15 and 50 per 100 000 individuals, with a female:male ratio of 6-10:1 in the age group between 15 and 40 yrs. The prevalence of systemic sclerosis is lower, however, varying significantly between different studies and countries. The prevalence of overlap syndromes, especially mixed connective tissue disease, is unknown, and polymyositis and dermatomyositis are regarded as very rare rheumatic diseases. Though the classification criteria for the connective tissue disorders have not been developed for the purpose of diagnosing an individual patient, these criteria still are the most valuable tool for the identification of patients with systemic rheumatic diseases such as connective tissue disorders.