A decrease in renal function leads to the accumulation of various uremic toxins (UTs) which exert unfavorable physiological effects on the body. Indoxyl sulfate (IS), a tryptophan-derived UT, is known to closely associate with the progression of cognitive disorders (CD) including dementia, in addition to chronic kidney disease (CKD) and cardiovascular events. It is, therefore, important to assess blood IS levels in CKD patients with CD. In this study, we assayed serum IS levels in 37 residents who had been admitted to a geriatric health services facility with stage G3b to G5 CKD and evaluated the correlation between serum IS levels and estimated glomerular filtration rate (eGFR). Eighteen out of 37 residents were considered to suffer dementia. When plotting all serum IS levels against eGFR, a weak but significant correlation was observed with a regression coefficient (r) of -0.420. In the non-dementia group, the correlation between serum IS levels and eGFR became stronger (r = -0.720). However, no correlation was observed in the dementia group. At CKD stage G3b, mean serum IS level was higher in the dementia group than in the non-dementia group. These results suggest that eGFR becomes a good marker to predict serum IS level in the case of CKD patients without dementia, but not in those with dementia. Therefore, direct monitoring of serum IS level is essential to assess the onset and/or progression of dementia in the elderly, irrespective of CKD stages.