Congenital stenosis of the aqueduct of Sylvius is reported to be associated with sella turcia enlargement and clinical and laboratory abnormalities of the hypothalamic-pituitary-target-organ axis. It is a surgically reversible lesion. In the present report, 3 female patients with this lesion were studied with tests of basal endocrine function, as well as insulin tolerance tests, response to metyrapone and determination of circadian periodicity of plasma cortisol levels. In one patient all testing was normal and no surgery was performed. In 2 other patients the insulin tolerance test revealed either abnormal cortisol or growth hormone responses and in one patient urinary gonadotrophins were absent. All tests became normal post-operatively although in one instance not completely so until 5 years after surgery.