The purpose of this study was to determine to what extent nurses practising in long-term care settings are able to detect delirium among elderly clients. This was a prospective clinical study, with two measurements taken seven days apart. The population studied consisted of 156 elderly people with dementia, and 32 nurses. The findings showed that these nurses had difficulty detecting delirium in this clientele. Their detection sensitivity varied from 7.3% to 15.5% and its specificity ranged from 95.2% to 100%. The probability of detection was higher when the elderly person experienced pain accompanied by delirium. For clients' well-being and the credibility of the profession, it appears crucial that steps be taken immediately to enable nurses to become fully competent in assessing seniors' mental states. Training in the clinical examination of mental states, clinical mentoring and the integration of an assessment tool into nursing practice are among the means suggested.