To the best of our knowledge, the association between COPD and chronic renal failure (CRF) has never been assessed. Lean mass is frequently reduced in COPD, and the glomerular filtration rate (GFR) might be depressed in spite of normal serum creatinine (concealed CRF). We investigated the prevalence and correlates of both concealed and overt CRF in elderly patients with COPD. We evaluated 356 consecutive elderly outpatients with COPD enrolled in the Extrapulmonary Consequences of COPD in the Elderly Study and 290 age-matched outpatients free from COPD. The GFR was estimated using the Modification of Diet in Renal Disease Study Group equation. Patients were categorized as having normal renal function (GFR > or = 60 mL/min/1.73 m(2)), concealed CRF (normal serum creatinine and reduced GFR), or overt CRF (increased serum creatinine and reduced GFR). Independent correlates of CRF were investigated by logistic regression analysis. The prevalence of concealed and overt CRF in patients with COPD was 20.8% and 22.2%, respectively. Corresponding figures in controls were 10.0% and 13.4%, respectively. COPD and age were significantly associated with both concealed CRF (COPD: odds ratio [OR] = 2.19, 95% CI = 1.17-4.12; age: OR = 1.06, 95% CI = 1.04-1.09) and overt CRF (COPD: OR = 1.94, 95% CI = 1.01-4.66; age: OR = 1.06, 95% CI = 1.04-1.10). Diabetes (OR = 1.96, 95% CI = 1.02-3.76), hypoalbuminemia (OR = 2.83, 95% CI = 1.70-4.73), and muscle-skeletal diseases (OR = 1.78, 95% CI = 1.01-3.16) were significant correlates of concealed CRF. BMI (OR = 1.05, 95% CI = 1.01-1.10) and diabetes (OR = 2.25, 95% CI = 1.26-4.03) were significantly associated with overt CRF. CRF is highly prevalent in patients with COPD, even with normal serum creatinine, and might contribute to explaining selected conditions such as anemia that are frequent complications of COPD.