A method of assessing the ongoing kidney status of a mammal afflicted with or at risk of developing chronic renal injury or disease, including chronic renal failure (CRF) by detecting the quantity of Neutrophil Gelatinase-Associated Lipocalin (NGAL) in urine, serum or plasma samples at discrete time periods, as well as over time. Incremental increases in NGAL levels in CRF patients over a prolonged period of time are diagnostic of worsening kidney disease. This increase in NGAL precedes and correlates with other indicators of worsening chronic renal disease or CRF, such as increased serum creatinine, increased urine protein secretion, and lower glomerular filtration rate (GFR). Proper detection of worsening (or improving, if treatment has been instituted) renal status over time, confirmed by pre- and post-treatment NGAL levels in the patient, can aid the clinical practitioner in designing and/or maintaining a proper treatment regimen to slow or stop the progression of CRF.