Equations for estimated glomerular filtration rate (eGFR) using levels of serum creatinine (Scr) or cystatin C may introduce bias when applied to populations with different ancestry, body type, or diet than the population where the equation was derived. The accuracy of eGFR equations has never been evaluated in persons with or at risk for Mesoamerican nephropathy (MeN), a syndrome of chronic kidney disease of unknown cause (CKDu) usually affecting young men from agricultural areas in Mesoamerica who are of mixed ancestry, perform strenuous manual labor, and live in poverty.