(en) The prevalence of chronic kidney disease (CKD) is increasing with aging. Approximately 1 out of 2 persons with CKD meets the criteria of CKD (an eGFR<60 ml/min). In this thesis we try to answer two research questions: “What are the consequences of an impaired eGFR?” and “What method or equation do we use to estimate or measure the GFR in older persons. In a first “epidemiological” part the outcomes of CKD and the factors predicting higher chances for important outcomes are analyzed. These analyses are performed on the data of 2 prospective population based studies (BELFRAIL and Leiden85+) and a primary care morbidity registry (Intego). The second part of this thesis reports our search for a reliable method to measure the GFR in older persons. This search included a systematic literature search, an analysis of the differences between different GFR estimating equations, an experimental part were the GFR in older persons was measured using the clearance of iohexol and the protocol of a new study.