Optical coherence tomography is a simple, high-resolution technique to quantify the thickness of retinal nerve fiber layer (RNFL). Previous studies have shown that degenerative changes occur in optic nerve fibers and are manifested as thinning of RNLF in patients with Alzheimer's disease (AD). However, there are no studies on the thickness of the RNLF in other types of dementia, such as dementia with Lewy bodies and dementia associated with Parkinson's disease. In this study, patients fulfilling diagnostic for AD (n = 10), dementia with Lewy bodies (n = 10), dementia associated with Parkinson's disease (n = 10), and cognitively normal age-matched controls (n = 10) underwent optical coherence tomography examinations to measure RNLF thickness. There was a significant decrease in RNLF thickness in each type of dementia compared to the control group (Mann-Whitney test, all p < 0.001). Although patients with dementia with Lewy bodies may have a greater thinning than both patients with AD and dementia associated with Parkinson's disease, the differences were statistically nonsignificant (Kruskal-Wallis test, p = 0.525). The thickness of the RNLF correlated significantly (p < 0.001) with both the Mini-Mental State Examination and the Mattis Dementia Rating Scale scores in all types of dementia; that is to say, the greater the cognitive deterioration, the greater the reduction of thickness of the RNLF. The findings from this study show that retinal involvement measured by optical coherence tomography may also be present in non-AD dementias.