Daily activities often depend on estimates and guesses. In this study 38 patients with probable Alzheimer's Disease were assessed with a Cognitive estimation task (Test zum kognitiven Schätzen) and their answers were compared with healthy control subjects. The Alzheimer's Disease patients showed significant deficits in cognitive estimation in all tested dimensions (size, weight, quantity, distance, and time) in both categories „perceptual” and „semantic”. In Alzheimer's Disease patients there were differences between the dimensions (quantity was easiest), while for control subjects, performance in different dimensions of estimation was equal. A model of cognitive estimation is proposed which can explain possible underlying deficits in Alzheimer's Disease patient's cognitive estimation.