The connective tissue diseases include a heterogeneous group of conditions characterized by a wide variety of signs and symptoms. There also exists, however, a group of systemic autoimmune disorders with signs and symptoms not sufficiently evolved to fulfill any of the accepted classification criteria for the defined connective tissue diseases. These conditions have been defined as undifferentiated connective tissue syndromes, latent lupus, incomplete lupus, and undifferentiated connective tissue diseases. After an analysis of the existing literature, we discuss the possible significance of the undifferentiated diseases among the already defined connective tissue diseases and make a proposal for preliminary classification criteria for these conditions.