The authors discuss reabsorption of cerebrospinal fluid in relation to the post-mortem findings in two children, one with total agenesis of the Pacchionian system accompanied by hydrocephalus, and the other with a subtotal agenesis and no hydrocephalus. Case 1 is the only known documented case of total agenesis of the Pacchionian system and gives credence to the idea that an impaired reabsorption of cerebrospinal fluid at the level of the Pacchionian system is a cause of hydrocephalus. The patient in Case 2 showed only two small areas of arachnoid granulations containing a few flattened, microscopic villi of normal cellularity.