The purposes of this article are, first, to report a case of large retroperitoneal chylous cyst and, second, to advance a new explanation for the formation of a cyst of this sort on the basis of experimental work on lymphatics. REPORT OF CASE G. C., a white man 47 years old and a tailor by occupation, came to the Johns Hopkins Hospital with the complaint of rupture. Previous History. —His general health had been good. There had been no abdominal symptoms except occasional postprandial eructations and moderate constipation which required the use of cathartics twice a week. There had been no jaundice or melena. When 19 years old, he had had gonorrheal infection. Present Illness. —About seventeen years before admission, he noticed a small swelling about the size of the end of his thumb in the midline of the epigastrium. The mass gradually increased in size without producing any symptoms