Over the past decade it has become evident that a substantial minority of patients with primary dementing diseases, particularly those presenting in the presenium, have dementia of frontal lobe type (DFT) due to non-Alzheimer's pathology. Although post-mortem remains the only method of definitive diagnosis, DFT and Alzheimer's disease (AD) can, we would claim, be separated with a high degree of certainty based on a combination of informant history, neuropsychology and neuroimaging. In DFT, changes in personality, motivation, social interaction and organisational abilities, in the presence of well preserved memory and visuospatial abilities, are characteristic. The lack of insight emphasises the need for independent information particularly as patients may perform normally on bedside (and more sophisticated) tests of cognition. Psychiatric features especially mood disturbance appear to be common, but their prevalence remains to be established. In contrast, AD is a progressive amnestic disorder with episodic and semantic memory deficits, followed by breakdown in other attentional, perceptual and visuo-spatial abilities, which reflects the major locus of pathology in AD, namely the medial temporal lobe. These features are illustrated by reference to a longitudinal study of 52 patients with minimal to moderate AD. In addition, we shall describe the results of retrospective and prospective neuropsychiatric studies of a group of DFT patients.