The nature and prevalence of misdemeanor in patients with dementia due to frontotemporal lobar degeneration has been described in a few case reports and in two small U.S. studies. Our clinical impression suggests that antisocial and aggressive behaviour are relatively frequent in this patient population. The objective of the present study was to verify this observation. For this purpose we developed a standardized questionnaire on misdemeanor in Frontotemporal Dementia. Using this instrument caregivers of 30 patients with Frontotemporal Dementia (FTD), 11 patients with Semantic dementia (SD) and 33 patients with Alzheimer-type dementia (AD) were interviewed. The interview included questions about theft, burglary, damaging other peoples' belongings, verbal or physical offence, bodily harm, drug abuse and use of weapons. Questions about the frequency of criminal behaviour, the amount of damages and consequences if applicable completed the questionnaire. Misdemeanor was found in half of the patients with FTD (15 out of 30) and in 7 out of 11 patients with SD, but only in one out of 33 patients with AD. The most frequent type of inappropriate behaviour was theft (13 patients), particularly shoplifting. 8 patients with FTD, 1 patient with SD and 1 patient with AD entered someone else's house without permission. 10 patients with FTD and 3 patients with SD but none of the patients with AD had physically threatened spouses, relatives or strangers. In one case another person was hurt.