BACKGROUND: Occupational heat stress among wildland firefighters (WFFs) performing arduous work or working in hot work environments may cause kidney dysfunction and injury. METHODS: Kidney function and injury biomarkers (serum and urine) were measured among 39 WFFs pre- and post-fire season in 2018-2019. The same biomarkers were measured in 19 of these 39 WFFs over 3 days during a 2019 mid-season fire incident. Median differences in biomarker concentrations across the fire season and across the mid-season incident were evaluated using the Sign test. The primary outcome of interest was the cystatin C-based estimated glomerular filtration rate (eGFR(cys)). RESULTS: The eGFR(cys) decreased (median difference = -5 mL/min/1.73 m(2); interquartile range [IQR] = -8, -2 mL/min/1.73 m(2); p = 0.008), and 53% of participants lost ≥ 2% of their body weight across the first day of the mid-season fire incident. Median eGFR(cys) did not decrease across the fire season (median difference = 0 mL/min/1.73 m(2); IQR = -5, 5 mL/min/1.73 m(2); p = 0.52). The albumin-creatinine ratio and the ratios of urine kidney injury molecule-1 and neutrophil gelatinase-associated lipocalin concentrations to urine osmolality increased across ≥ 1 day during the mid-season incident. CONCLUSIONS: A temporary decrease in kidney function and changes in biomarkers of kidney injury were observed during a wildland fire incident. Additional research is warranted to confirm these findings, assess associations with occupational heat stress, and determine whether persistent, clinically relevant kidney injury and dysfunction occur among WFFs over time. The findings also support the need for continued efforts to promote optimal hydration of WFFs.