Event-related potentials (ERPs) were recorded from 16 healthy young (mean age 21 years) and 16 healthy old subjects (mean age 64 years), and from 11 subjects with a diagnosis of Dementia of Alzheimer Type (DAT). The task requirement was to attend to a series of visually presented words so as to respond to occasional animal names. Non-animal names repeated after either a single or six intervening items. In the young subjects ERPs evoked by repeated words displayed a widespread, sustained positive-going shift relative to ERPs evoked by first presentations (the ERP repetition effect). This effect onset around 220 msec and did not differ as a function of inter-item lag. Other than for a delay in onset of approximately 80 msec, the ERP repetition effect in the healthy old group was in all respects equivalent to that of the young subjects. The ERP repetition effects in the DAT patients were statistically indistinguishable from those of an appropriately matched sub-set of the healthy old subjects. These results indicate that the ERP repetition effect remains robust in subjects in whom explicit memory has declined as a result of normal aging or DAT. Thus they suggest that the effect reflects processes independent of those underlying explicit memory, and that it may index a form of memory relatively unaffected by the pathology underlying DAT.