Chronic kidney disease (CKD) afflicts up to 20 million people in the United States, but little is known about their health care costs. The authors analyzed costs and resource use associated with CKD by using National Kidney Foundation staging definitions. Patients insured through a large health maintenance organization with a laboratory finding of CKD (defined as estimated GFR between 15 and 90 ml/min per 1.73 m(2) in 1996 followed by a second GFR below 90 at the next creatinine measurement occurring at least 90 d later) were followed from 1996 for up to 66 mo. The final cohort included 13,796 persons with CKD and their age- and gender-matched controls; 1741 in stage 2; 11,278 in stage 3; and 777 in stage 4. Depending on stage, cases had 1.9 to 2.5 times more prescriptions, 1.3 to 1.9 times more outpatient visits, were 1.6 to 2.2 times more likely to have had an inpatient stay, and had 1.8 to 3.1 more stays than did controls. Total per patient follow-up costs were [$total, (95% CI) cases and controls, respectively] $38,764 (95% CI, 37,033 to $40,496) and $16,212 (95% CI, $15,644 to $16,780) in stage 2; $33,144 (95% CI, $32,578 to $33,709) and $18,964 (95% CI, $18,730 to $19,197) in stage 3; and $41,928 (95% CI, $39,354 to $44,501) and $19,106 (95% CI, $18,212 to $20,000) in stage 4. Cases with no CKD-related comorbidities had costs double that of controls with no CKD-related comorbidities, and comorbidities related to CKD were more costly to manage than CKD alone. Future research in this area could be usefully directed toward analyzing the clinical and economic consequences of better managing or preventing comorbidities in patients with CKD.