OBJECTIVE To assess the prevalence of cognitive impairment in persons with chronic kidney disease (CKD) and its relation to the severity of CKD. METHODS Cross-sectional study. METHODS University-affiliated ambulatory nephrology and dialysis practices. METHODS Eighty subjects with CKD Stages III and IV not requiring dialysis (CKD) and 80 subjects with CKD Stage V on hemodialysis (end-stage renal disease (ESRD)) with a mean age+/-standard deviation of 62.5+/-14.3. METHODS Three standardized cognitive tests, the Modified Mini-Mental State Examination (3MS), Trailmaking Test B (Trails B), and California Verbal Learning Trial (CVLT). Glomerular filtration rate was estimated in subjects with CKD using the six-variable Modification of Diet in Renal Disease equation. RESULTS There was a graded relation between cognitive function and severity of CKD. Mean scores on the 3MS, Trails B, and CVLT immediate and delayed recall were significantly worse for subjects with ESRD than for subjects with CKD or published norms (P<.001 for all comparisons). Scores on the Trails B (P<.001) and CVLT immediate (P=.01) and delayed (P<.001) recall were significantly worse for subjects with CKD not requiring dialysis than for published norms. In addition, the fraction of subjects with impairment on the 3MS and Trails B increased with decreasing kidney function. CONCLUSIONS Cognitive impairment is associated with the severity of kidney disease. Further studies are needed to determine the reasons for cognitive impairment in subjects with CKD and ESRD.