Background: The incidence of acute kidney injury (AKI) is increasing. AKI is currently recognised as an inducer of chronic kidney disease (CKD) and this is known as the ‘AKI–CKD transition’. This study aimed to evaluate the rate of decline in estimated glomerular filtration rate (eGFR) associated with AKI events in individuals with and without pre-existing CKD. Methods: Inpatients aged 18–80 years were retrospectively enrolled. AKI was diagnosed according to the kidney disease improving global outcomes (KDIGO) criteria using serum creatinine levels. Patients with a history of AKI events were divided into four groups according to eGFR before and after the AKI events. In each group, the eGFR levels after an AKI event were compared to those before the AKI event. Patients were further divided into eight groups according to clinical background based on underlying diseases, medications, and surgical history. Results: We analysed data from 9651 patients with AKI. Not surprisingly, we found that eGFR levels during the first AKI event were significantly lower than levels before the event in each group. Furthermore, eGFR levels after the first AKI event were significantly lower than those before the first AKI event, and the eGFR levels after the second AKI event were significantly lower than those after the first AKI event. These trends were similar in each group irrespective of clinical background. Conclusions: Our study revealed that AKI events can cause a decline in kidney function and, as more AKI events occur, acceleration of this decline.