For those who have "true" dry eye - that is, aqueous deficient dry eye where the primary problem is lacrimal glands not doing their job - Sjögrens Syndrome is the most common cause. (For more on the different causes and types of dry eye, please see Dry Eye for Dummies.)
Chronic dry eye patients who have a very low Schirmer score may want to consider getting tested for Sjögrens Syndrome, particularly if they experience any of the other symptoms of SS.
Here is a definition of Sjögrens Syndrome taken from the Sjögren's Syndrome Foundation website:
"Sjögren's ("SHOW-grins") syndrome is a chronic disease in which white blood cells attack the moisture-producing glands. The hallmark symptoms are dry eyes and dry mouth, but it is a systemic disease, affecting many organs and may cause fatigue. It is one of the most prevalent autoimmune disorders, striking as many as four million Americans."