Twenty-two patients meeting the NINCDS-ADRDA diagnostic criteria for probable AD were included in the study, along with 10 matched controls. Praxic disturbances were investigated using eight tasks and the results were interpreted according to the neuropsychological model of Roy and Square modified by Rothi et al. (1988) [Aphasiology 21:381-388] which distinguishes a conceptual system concerned with knowledge of the action and function of gestures and a production system that effects gestures in the environment. Disturbance of the production system was found only in 17 patients. Disturbance of the production system was correlated to disturbance of verbal comprehension. The patients scored lower using the left hand than the right. Disturbance of the conceptual system was found in all patients and was not significantly correlated with other cognitive deficits. No significant difference in results was found according to the type of input to the conceptual system (visual or verbal). Deficits in tasks using real objects were correlated to disturbances of both the production and conceptual systems. Most patients performed poorly both in tasks exploring the conceptual system and in those exploring the production system. However, two patients performed badly in production tasks but had performances in the range of controls for conceptual tasks and one patient had the opposite pattern of dissociation. This provides evidence that the production and the conceptual system are independent. Impairment in the ability to perform everyday activities was correlated to disturbances of the conceptual system whereas poor performances in tasks exploring the production system or in using real objects were not.