The aim of this study was to compare the diagnostic profiles of patients with early (age<65 years) and late (age>or=65 years) onset of dementia in a memory disorders clinic in Japan. A total of 512 consecutive memory clinic patients were evaluated using clinical information and results of examinations. Diagnosis of dementia was made according to DSM-III-R, and that of subtypes according to standard diagnostic criteria. A total of 464 patients met the criteria for dementia. Amongst late-onset patients (n=430), Alzheimer's disease (AD) (48.1%) was the most frequent cause of dementia, followed by AD with cerebrovascular disease (CVD) (31.4%), vascular dementia (VaD) (9.1%), dementia with Lewy bodies (DLB) (3.7%), frontotemporal lobar degeneration (FTLD) (1.6%), and others (5.8%). On the contrary, amongst early onset patients (n=34), the most common dementia diagnosis was AD (38.2%), followed by VaD (23.5%), FTLD (14.7%), AD with CVD (5.9%), DLB (2.9%), and others (17.6%). FTLD and VaD were significantly more common in the early onset group. All patients, but one, with DLB and Parkinson's disease dementia were late-onset. The relative frequencies of AD, VaD, and DLB in our series are consistent with epidemiologic findings in several Western countries; however, the frequency of FTLD is not consistent with the previous findings presenting high frequency in late-onset patients in some Western countries.