By reviewing and collating data in a 2-step postal survey sent to all of the institutions for individuals with dementia in Ibaraki prefecture requesting information on early-onset dementia (EOD) cases, 617 subjects with EOD were identified. The estimated prevalence of EOD in the target population was 42.3 per 100,000. Of the illness causing EOD, vascular dementia was the most frequent followed by Alzheimer's disease, head trauma, dementia with Lewy body/Parkinson's disease with dementia, frontotemporal lobar degeneration, and other causes. On the other hand, hereditary diffuse leucoencephalopathy with spheroids (HDLS) is an autosomal-dominant central nervous system white matter disease with variable phenotypes. The onset of symptoms is usually in the fourth or fifth decade, progressing to dementia with death within 6 years. Recently, several mutations of the colony stimulating factor 1 receptor encoded by CSF1R segregating HDLS were identified. Since clinical presentations varied substantially within and across families with HDLS, CSF1R mutation carriers may be present in clinical series of Alzheimer's disease, frontotemporal lobar degeneration, corticobasal syndrome, multiple sclerosis, CADASIL, Parkinson's disease and ischemic stroke with additional white matter changes, all causing EOD. In the differential diagnosis of EOD, we should always consider HDLS and if necessary perform CSF1R gene analysis.