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Abstract INTRODUCTION Some people with dementia develop changes in behaviour and cognition that may lead to interactions with police or the legal system. However, large, prospective case–control studies examining these behaviours are lacking. METHODS One hundred and forty‐four people with dementia and 53 controls completed the Misdemeanours and Transgressions Screener. RESULTS Criminal risk behaviours were reported in: 65.6% of behavioural‐variant frontotemporal dementia, 46.2% of right‐lateralised semantic dementia, and 27.0% of Alzheimer's disease patients. In 19.1% of patients these behaviours led to contact with police or authority figures. Compared to controls, people with dementia showed higher rates of physical assault (p = 0.024), financial/professional recklessness (p = 0.009), and inappropriate behaviours (p = 0.052). DISCUSSION Criminal risk behaviours are common across dementia subtypes and may be one of the first clinical signs of frontotemporal dementia. Further research to understand how to balance risk minimisation with an individual's liberties as well as the inappropriate criminalisation of people with dementia is needed. Highlights The Misdemeanours and Transgressions Screener is a new tool to assess criminal risk behaviours. Forty‐seven percent of patients with dementia show criminal risk behaviour after dementia onset. Behaviours included verbal abuse, traffic violations, physical assault. New onset of criminal risk behaviours >50 years is a clinical sign for frontotemporal dementia.