THE CASES OF a father and his son who were diagnosed with pathological
ly confirmed colloid cysts of the third ventricle are presented. The f
amilial occurrence of this tumor is rare and suggests that genetic fac
tors may play a role in its formation. Consistent with the concept tha
t the cyst originates as a developmental abnormality, it is associated
with a variety of congenital defects. The conditions that are associa
ted with these tumors are discussed.