The term undifferentiated connective tissue diseases is used to define conditions characterized by the presence of signs and symptoms suggestive of a systemic autoimmune disease that do not satisfy the classificative criteria for defined connective tissue diseases (CTD) such as systemic lupus erythematosus (SLE), Sjögren's syndrome (SS), rheumatoid arthritis (RA) and others. A small percentage of patients presenting with an undifferentiated profile will develop during the first year follow up of a full blown CTD, however an average of 75% will maintain an undifferentiated clinical course. These patients may be defined as having a stable undifferentiated connective tissue diseases (UCTD). The most characteristic symptoms of UCTD are represented by arthritis and arthralgias, Raynaud's phenomenon, leukopenia, while neurological and kidney involvement are virtually absent. Eighty percent of these patients have a single autoantibody specificity, more frequently anti-Ro and anti-RNP antibodies. Stable UCTD are considered as distinct clinical entities and therefore it has been proposed to define those conditions as UCTD. Classificative criteria have also been proposed and a work to better define them is still under way.