An attempt is made to account for the ability of severely amnesic patients to learn and retain certain tasks but not others. The hypothesis is advanced that the amnesic subject can show learning through facilitation by repetition or of simple S-R relationships not requiring cognitive mediation. Three examinations are reported. The first demonstrates that the amnesic subjects have adequate speed of access to semantic and phonological knowledge and show the same degree of improvement on retesting as controls. The second and third experiments examine different degrees of cognitive mediation in paired-associate learning. Amnesic subjects are differentially impaired relative to controls on "distant" pairs and on those that, by hypothesis, would benefit from cognitive mediation. The results are discussed in terms of a disconnection syndrome. It is suggested that at the functional level, a "cognitive mediational memory system" and a semantic memory system are disconnected in the amnesic patient. Neuropathological evidence suggests that temporal lobe and frontal lobe structures are disconnected by pathways in the fornix-mammillary body route.