The existence of patients with signs and symptoms suggestive of a systemic autoimmune disease but not fulfilling the classification criteria for defined diseases is a common experience in clinical practice. The first description of these diseases dates back to 1980, when LeRoy proposed the term "Undifferentiated Connective Tissue Syndromes" (UCTS) to define the early phases of connective tissue diseases that at their onset are undefined, as opposed to mixed or overlapping syndromes in which the disease appears well defined. LeRoy also raised the fundamental question as to whether a proportion of patients with an undifferentiated profile may see their condition remain undifferentiated or even experience a remission of all pathologic features, rather than evolving to a definite connective tissue disease (CTD). Many studies conducted on undifferentiated diseases have shown that up to 75% of patients will not develop a defined CTD and that these conditions exhibit typical clinical and serological manifestations and a good prognosis. We and others have defined these conditions as Undifferentiated Connective Tissue Diseases (UCTD) and it is our opinion that the UCTD represent distinct clinical entites. UCTD could offer an ideal clinical model for the study of single autoantibody specificities, the effects of various factors (such as pregnancy) on the disease course, and the general pathogenesis of autoimmune conditions. Before they can be so used, more must be discovered about their nature and characteristics, and studies will be necessary to improve the sensitivity and specificity of the existing preliminary classification criteria.