BACKGROUND: Acute kidney injury (AKI) is a common and serious condition defined by a rapid decline in kidney function. Data on changes in long-term kidney function following AKI are sparse and conflicting. Therefore, we examined the changes in estimated glomerular filtration rate (eGFR) from before to after AKI in a nationwide population-based setting. METHODS: Using Danish laboratory databases, we identified individuals with first-time AKI defined by an acute increase in plasma creatinine (pCr) during 2010 to 2017. Individuals with three or more outpatient pCr measurements before and after AKI were included and cohorts were stratified by baseline eGFR (≥/<60 mL/min/1.73 m2). Linear regression models were used to estimate and compare individual eGFR slopes and eGFR levels before and after AKI. RESULTS: Among individuals with a baseline eGFR ≥60 mL/min/1.73 m2 (n = 64 805), first-time AKI was associated with a median difference in eGFR level of -5.6 mL/min/1.73 m2 [interquartile range (IQR) -16.1 to 1.8] and a median difference in eGFR slope of -0.4 mL/min/1.73 m2/year (IQR -5.5 to 4.4). Correspondingly, among individuals with a baseline eGFR <60 mL/min/1.73 m2 (n = 33 267), first-time AKI was associated with a median difference in eGFR level of -2.2 mL/min/1.73 m2 (IQR -9.2 to 4.3) and a median difference in eGFR slope of 1.5 mL/min/1.73 m2/year (IQR -2.9 to 6.5). CONCLUSION: Among individuals with first-time AKI surviving to have repeated outpatient pCr measurements, AKI was associated with changes in eGFR level and eGFR slope for which the magnitude and direction depended on baseline eGFR.