It would not require a very gross disease to occlude the tiny channel of communication between the third and fourth ventricles of the brain. Actually, clinical symptoms and even death may result from a process so minute that it may be impossible to recognize it during the life of the patient, and its identity may be established after death only by use of the microscope. Particular interest, therefore, is attached to such cases because of this difficulty in clinical and in pathologic diagnosis. It is not intended to describe all of the conditions producing occlusion of the aqueduct, but a small group of patients have been selected wherein the difficulties mentioned are exemplified. REPORT OF CASES Case 1.—History. —A girl, aged 11, was brought to the Mayo Clinic on Jan. 11, 1918, because of headache, backache and difficulty in walking. Five years before, and for no apparent reason, she had