BACKGROUND: Alzheimer's disease (AD) is a common cognitive disease that can progress at an accelerating rate. Even with early diagnosis, the families might not recognize AD progressing unless behavioural and psychological symptoms of dementia (BPSD) develop. In many cases, discrepancies could exist between family-assessed AD stage and diagnosed AD stage. This study explored such discrepancies and potential clinical implications. METHODS: Participants were 161 new outpatients with AD or mild cognitive impairment at four memory clinics whose AD stage was diagnosed using the Revised Hasegawa Dementia Scale (HDS-R) and Mini-Mental State Examination (MMSE). We classified patients into four groups according to AD severity. Family members completed the Functional Assessment Staging (FAST) scale during an interview. We then assigned patients to three groups according to discrepancies between family-assessed and diagnosed AD stage. Families also completed the Neuropsychiatric Inventory Questionnaire (NPI-Q), which assesses 12 neuropsychiatric domains, in order to examine the presence of BPSD in relation to AD stage. RESULTS: Most families (74%-80%) assessed patients as having milder AD than the diagnosed stage. NPI-Q scores and duration of education significantly affected discrepancies with HDS-R and MMSE scores. The NPI-Q domains of anxiety, apathy/indifference, aberrant motor behaviours, and appetite/eating disturbance significantly affected family-assessed FAST. Families of patients with more years of education assessed the AD stage as more advanced than the diagnosed stage. Surprisingly, living together did not significantly affect the discrepancy. CONCLUSIONS: Most families assessed AD as milder than the clinically diagnosed AD stage. In addition, high NPI-Q scores and more years of school education significantly affected the discrepancy. Family-assessed FAST was significantly affected by the NPI-Q domains of anxiety, apathy/indifference, aberrant motor behaviours, and appetite/eating disturbance. These results suggest that obvious BPSD are significant factors for Japanese families to recognize AD progress.