The determination of serum creatinine and serum urea nitrogen levels is of great value in helping to ascertain the renal function in the clinical setting. These two serum determinations are best viewed in concert, observing their absolute levels as well as their relation to one another. The serum creatinine level is less influenced by extra-renal factors than is the serum urea nitrogen level, and is the more accurate test. Reproducibility of measurement is within 2 per cent. When the test results return, there are a number of questions to be asked: What is the normal range for the laboratory which analyzed the serum? Are the levels real; i.e., could the elevated levels be factitious? Are there extrarenal etiologies for the abnormal levels? What is the BUN to creatinine ratio? If the abnormal findings are secondary to intrinsic renal disease, what other tests will help to determine the etiology of the renal disease? The laboratory assessments of BUN levels and blood creatinine levels are "standard fare" in the assessment of renal function. They are relatively low-cost tests, are available in any standard hospital laboratory, and are relatively easy to run. They are essential in the assessment of renal function in the Emergency Department.