A case of amnesia after excision of the septum pellucidum. Tumours of the septum pellucidum (SP) are rare and seldom associated with memory impairment either before or after operation. A patient is described who developed amnesia after transcallosal excision of a tumour of the SP. Radiology did not show any major lesion of the brain areas traditionally associated with amnesia. Because septal nuclei could have been damaged during surgery their possible role in memory functions is discussed. 