In clinical practice and for scientific research a method is needed for the assessment of changes in functioning in daily life of dementia patients living at home. As existing methods have their limitations, a questionnaire was developed (the Interview for Deterioration in Daily life in Dementia: IDDD), to be answered by caregivers. The questionnaire concerns self-care and complex activities, that are usually performed by men as well as women. Attention is paid to the difference between lack of initiative and impaired performance itself. Caregivers are interviewed in a structured interview. A study among 30 caregivers of dementia patients living at home, showed the relevance and usefulness of the questions. The questionnaire shows high internal consistency, which may be the result of our structured interview technique.