Patients with Alzheimer's disease (AD) are often impaired on certain forms of implicit memory, such as word-stem completion priming (WSCP). Lesion data suggest that deficient WSCP may be associated with abnormal functioning in the posterior neocortex. Using positron emission tomography (PET), we here provide direct support for this view. Compared with normal old adults, AD patients showed reduced priming on a word-stem completion task. The normal old showed decreased activity in right occipital cortex (area 19), whereas the AD patients showed increased activity in this region during priming. To the extent that decreased activity during priming reflects an experience-dependent reduction of the neuronal population involved, these results indicate that shaping of the relevant neurons is slower in AD, possibly as a result of inadequate initial stimulus-processing.