The relationship between the rheumatoid diseases and diseases of the so-called collagen-vascular group is both close and uncertain. Many of the clinical symptoms found classically in one disease or syndrome may be found in another; the difference is on emphasis of certain aspects over others. This similarity extends to laboratory tests and makes it even more difficult to place borderline or problem cases into one group or the other. Until the exact etiology of each disease is known, overlap in laboratory test results is likely to continue. Fortunately, most patients can be assigned to satisfactory diagnostic categories using available clinical and laboratory data.