To examine how the severity of dementia affects attitudes to treatment preferences in a lay group of community residents and a group of medical care professionals who provide direct care to dementia affected patients. The participants were 259 community residents aged between 40 and 65, and 217 care professionals working at nursing homes or group homes. Respondents were randomly assigned to one of two scenarios involving moderate or severe dementia and each was asked questions about their preferences and attitudes to the employment of eight types of active treatments (ATs) to deal with a newly acquired illness as well as eight types of life-sustaining treatment (LST). Among the community residents, there were no significant differences in preferences toward any treatment items between the moderate dementia and severe dementia scenarios. Similarly, care professionals showed no significant differences in attitudes toward 15 of the 16 treatment items. The community residents had more negative attitudes than care professionals in attitudes to all types of LST, including four variations of AT that have a good chance of success. After dementia deprives an individual of decision-making capacity, progress of the disease has little effect on both community residents and care professionals' preferences. When discussing about end-of-life decision-making, care professionals need to be careful about the gaps in perception of good chance treatments with patients.