Urea is a waste product of protein metabolism, and along with creatinine is a commonly-ordered blood test that is used to measure and track kidney function. In routine clinical practice, the creatinine concentration is used to compute estimated glomerular filtration rate (eGFR), which is a well-validated method for measuring the percentage of kidney function. Higher creatinine levels reflect poor kidney function (and lower eGFR values) and are associated with a higher risk of death. We believe that urea, and possibly the ratio of urea/creatinine, are better indicators of the adverse risk that is associated with reduced kidney function based on emerging evidence that urea may itself be toxic to human cells. The goal of this study is to evaluate the associations of urea, creatinine, and urea:creatinine ratio with mortality in the general population. We will determine whether urea is superior to creatinine concentrations and other blood measures of kidney function in predicting the adverse outcome of death. Using data from the Third National Health and Nutrition Examination Survey, (NHANES III), we will test our hypothesis in over 15,000 individuals, representative of the general US population, who have follow up data for 15 years. The results of this study, if positive, will be of value to health care practitioners in assessing the overall health of individuals and in identifying those who have the greatest risk for reduced survival.