In 1954 Scoville described a grave loss of recent memory which he had observed as a sequel to bilateral medial temporal lobe resection in one psychotic patient and one patient with intractableseizures. In both cases the operations had been radical ones, undertaken only when more conservative forms of treatment had failed. The removals extended posteriorly along the mesial surface of the temporal lobes for a distance of approximately 8 cm from the temporal tips and probably destroyed the anterior two-thirds of the hippocampus and hippocampal gyrus bilaterally, as well as the uncus and amygdala. The unexpected and persistent memory deficit which resulted seemed to us to merit further investigation. We have therefore carried out formal memory and intelligence testing of these two patients and also of eight other patients who had undergone similar, but less radical, bilateral medial temporal lobe resections. The present paper gives the results of these studies which point to the importance of the hippocampal complex for normal memory function. Whenever the hippocampus and hippocampal gyrus were damaged bilaterally in these operationssome memory deficit was found, but not otherwise. We have chosen to report these findings in full, partly for their theoretical significance, and partly as a warning to others of the risk to memory involved in bilateral surgical lesions of the hippocampal region.