The purpose of this study was to investigate and analyze the moral tension that exists in the care for demented nursing home patients, between the principle of respect for autonomy and the value that is attached to respect for the subjective world of the patient. To this end an ethnographical field study was carried out by two researchers in two Dutch nursing homes. Among the central topics that evolved were the different moral problems that nurses experience concerning truth telling and acting truthfully in relation to demented patients. In situations unrelated to the dementia and its diagnosis, the right to be informed is in principle respected, even if the information is sometimes painful. More specific questions of demented patients about their situation are a regular cause of embarrassment for their carers, who rely on various treatment strategies to deal with such questions. These strategies are often successful. However, when they fail, the nurses are faced with a problem they cannot solve, namely the loss of a common shared world and the resulting unmentionable truth about the diagnosis of dementia, as objective basis and legitimization for their approach to the demented patient. We conclude that in the training and professional support given to nurses, more attention should be paid to (awareness of) the moral problems that arise from this loss of a common shared world, so that they can react to the subjective world of demented patients without feeling that they are deceiving them.