BACKGROUND: The Brain in Kidney Disease (BRINK) Study aims to identify mechanisms that contribute to increased risk for cognitive impairment in patients with chronic kidney disease (CKD). We describe the rationale, design, and methods of the study and report baseline recruitment and cognitive function results. STUDY DESIGN: Longitudinal observational cohort study of the epidemiology of cognitive impairment in CKD. The primary aim is to characterize the association between (1) baseline and incident stroke, white matter disease, estimated glomerular filtration rate (eGFR), inflammation, microalbuminuria, and dialysis initiation and (2) cognitive decline over 3 years in a CKD cohort with a mean eGFR<45mL/min/1.73m(2). SETTING & PARTICIPANTS: Community-dwelling participants 45 years or older recruited from 4 health systems into 2 groups: reduced eGFR, defined as eGFR<60mL/min/1.73m(2) (non-dialysis dependent), and control, defined as eGFR>/=60mL/min/1.73m(2). PREDICTOR: eGFR group. OUTCOMES: Performance on cognitive function tests and structural brain magnetic resonance imaging. MEASUREMENTS: Sequential cognitive and physical function testing, serum and urine biomarker measurement, and brain magnetic resonance images over 3 years. RESULTS: Of 554 participants, mean age was 69.3 years; 333, 88, and 133 had eGFRs<45 (non-dialysis dependent, nontransplantation), 45 to <60, and >/=60 (controls) mL/min/1.73m(2), respectively. Mean eGFR in reduced-eGFR participants was 34.3mL/min/1.73m(2). Baseline cognitive performance was significantly associated with eGFR in all domains except language. Participants with eGFRs<30mL/min/1.73m(2) performed significantly worse than those with eGFRs>/=30mL/min/1.73m(2) on tests of memory, processing speed, and executive function. Participants with reduced eGFRs overall scored worst on the Immediate Brief Visual-Spatial Memory Test-Revised. LIMITATIONS: Healthy cohort bias, competing risk for death versus cognitive decline. CONCLUSIONS: Cognitive function was significantly worse in participants with eGFRs<30mL/min/1.73m(2). Future BRINK analyses will measure risk factors for cognitive decline using the longitudinal data.