We studied the correlation between the caregiver's feelings of burden and the cognitive, behavioral and functional impairment of demented patients. We attempted to show the influence of caregiver's feelings of burden on their perception of the patients' functional status and to establish the predictors of feelings of burden. Twenty-five probable Alzheimer disease patients (NINCDS-ADRDA criteria) were assessed on cognitive measures and functional status (DAFS). The caregiver's index of burden (CIB), obtained from an adapted version of the Zarit Burden Interview, was correlated with the caregiver's report on the patient's instrumental activities of daily living (IADL) and physical self-maintenance functions (ADL); total and partial DAFS scores, and patient's cognitive and behavioral problems rated with the Functional Dementia Scale (FDS). The CIB correlated with the caregiver's report on the patient's behavioral disturbances (r = 0.71, p < 0.001) and physical self-maintenance activities (r = 0.62, p < 0.001), but not with cognitive impairment, IADL and the DAFS. The patients showed better functional performance in the direct assessment than in that reported by their caregivers (mainly in shopping, feeding, dressing, and ambulation). Thus, burden may foster a growing intolerance of the caregiver, inducing an underestimation of the patient's actual functional competence. The caregiver's report on the patient's behavioral problems was the best predictor of feelings of burden (F = 19,9, p < 0.001). We emphasized the utility of the DAFS for a more objective assessment of functional status, treatment of the patient's behavioral problems to lessen the caregiver's feeling of burden, and the assessment and control of the caregiver's burden to highlight the need for medical assistance or support groups.