The frontotemporal dementias are characterized by behavioral and language disorders, instead of the prominent memory loss that characterizes Alzheimer’s disease. Their loss of social intelligence, the severe apathy or des inhibition does differentiate them to the courtesy of most Alzheimer patients. Three selected cases are reported: the first one was characterized by apathy, the second by loss of the control of impulses, and the third one by complex stereotypic behavior. The diagnostic difficulty is remarked with the initial misdiagnosis in each case, and the clinical aspects are exposed.