d-Serine, a rare enantiomer of serine, is a biomarker of kidney disease and function. The level of d-serine in the human body is precisely regulated through the urinary clearance of the kidney, and its clearance serves as a new measure of glomerular filtration rate with a lower bias than creatinine clearance. d-Serine also has a direct effect on the kidneys and mediates the cellular proliferation of tubular cells via mTOR signaling and induces kidney remodeling as a compensatory reaction to the loss of kidney mass. In living kidney donors, the removal of the kidney results in an increase in blood d-serine level, which in turn accelerates kidney remodeling and augments kidney clearance, thus reducing blood levels of d-serine. This feedback system strictly controls d-serine levels in the body. The function of d-serine as a biomarker and modulator of kidney function will be the basis of precision medicine for kidney diseases.