Background: Although the Kidney Disease: Improving Global Outcomes (KDIGO) criteria are used to define acute kidney injury, the criteria have limitations for including 2 different serum creatinine criteria in stage 1. We hypothesized that there would be differences in clinical outcomes between the 2 subgroups of stage 1 acute kidney injury in patients undergoing cardiac or thoracic aortic surgery. Methods: We reviewed 2510 cases. Patients with KDIGO stage 1 were divided into 2 subgroups (stage 1a: 0.3 mg/dL or greater of absolute increase in serum creatinine, n = 376; and stage 1b: 50% or greater relative increase, n = 365). Propensity score analysis was performed between stage 1a and 1b groups, yielding 240 pairs. We compared the length of hospital stay, the incidence of cardiovascular complications, 5-year all-cause mortality between these subgroups. Overall survival was compared between the subgroups after propensity score matching. We performed sensitivity analysis for Acute Kidney Injury Network (AKIN) criteria. Results: Length of hospital stay and 5-year all-cause mortality were worse in patients with KDIGO stage 1b compared with stage 1a. Five-year patient survival was significantly worse in patients with stage 1b compared with stage 1a after matching (log rank test, P = .002). We found similar results regarding AKIN criteria. Subgroup analysis showed that the significant difference in survival existed only when baseline serum creatinine was 0.8 mg/dL or greater. Conclusions: The KDIGO or AKIN criteria for stage 1 acute kidney injury could be further divided into 2 substages with different severity of clinical outcomes. These modified criteria could give additional prognostic information in patients undergoing cardiac or thoracic aortic surgery.