Connective tissue diseases (CTDs) frequently present as undifferentiated disorders that may or may not develop into one of the well defined CTDs. The natural evolution of the early undifferentiated CTDs is not well known. Some years ago a cooperative study was started on 410 patients with early CTD; within this group, 48 patients had early scleroderma. Raynaud's phenomenon has been recognized as an early sign of CTD. Several studies on Raynaud's phenomenon patients have shown that abnormal nailfold capillary patterns have prognostic significance for evolution to a CTD, particularly scleroderma. Mixed connective tissue disease (MCTD) is the prototype of an overlap syndrome. The relevance of defining MCTD as a separate disease entity has been challenged. Follow-up studies show that many patients originally diagnosed with MCTD develop a definite CTD, particularly scleroderma or systemic lupus erythematosus, within a few years. In patients with MCTD, primary pulmonary hypertension is an important cause of death. This condition seems to be associated with anticardiolipin antibodies. A number of case reports suggest that immunosuppressive treatment is of benefit in this almost always fatal condition. The main reason for describing MCTD as a separate entity is its association with antibodies to U1-RNP. Recent studies have suggested that the production of anti-U1-RNP is an antigen-driven process, but the nature of the trigger for its production has not been elucidated. Retroviral proteins do not appear to be involved. Antibodies to the protein part of the U1-RNP complex and antibodies to the RNA component have been described. Levels of anti-RNA antibodies seem to parallel disease activity in MCTD patients.(ABSTRACT TRUNCATED AT 250 WORDS)