Background: Vasopressin is elevated in response to heat and dehydration and has been postulated to have a role in the chronic kidney disease of unknown origin being observed in Central America. The aims of this study were to examine whether the vasopressin pathway, as measured by copeptin, is associated with the presence of kidney dysfunction, and to examine whether higher fluid intake is associated with lower circulating copeptin and thereby preserves kidney health among sugarcane workers exposed to hot conditions. Methods: Utilizing a longitudinal study of 105 workers in Guatemala, we examined relationships between hydration indices, plasma copeptin concentrations, and kidney function markers at 3 times during the 6-month harvest. We also examined whether baseline copeptin concentrations increased the odds of developing an estimated glomerular filtration rate (eGFR) <60 mL/min/1.73 m<sup>2</sup>. Results: Copeptin concentrations were positively associated with serum creatinine (β 1.41, 95% CI 0.88–2.03) and negatively associated with eGFR (β –1.07, 95% CI –1.43 to –0.70). In addition, as workers improved their hydration (measured by increases in fluid balance), copeptin concentrations were reduced, and this reduction was associated with an improvement in kidney function. Conclusions: Results suggest that copeptin should be studied as a potential prognostic biomarker.