Objective: Acute kidney injury in the critically ill is an independent risk factor for adverse outcome. The magnitude of the impact of acute kidney injury on outcome, however, is still unclear. This study aimed to estimate the excess mortality attributable to acute kidney injury. Design: We performed a sequentially matched analysis according to the day of acute kidney injury diagnosis after ICU admission. Patients with acute kidney injury and those without acute kidney injury were matched according to age, sex, ICU admission diagnosis, Simplified Acute Physiology Score II without renal and age components, and the propensity to develop acute kidney injury at each of the four matching time points. Setting: Cohort of 16 participating ICUs from the prospective Finnish Acute Kidney Injury study. Patients: Cohort of 2,719 consecutive patients with either emergency admission or elective postsurgical patients with an expected ICU stay greater than 24 hours. Interventions: None. Measurements and Main Results: Of the 2,719 patients included in the study, acute kidney injury developed in 1,081 patients (39.8 ) according to the Kidney Disease: Improving Global Outcomes-definition during ICU treatment on days 1-5. Of these, 477 patients were successfully matched to 477 patients who did not develop acute kidney injury. The 90-day mortality of the matched patients with acute kidney injury was 125 of 477 (26.2 ) compared with 84 of 477 (17.6 ) for their matched controls without acute kidney injury. Thus, the absolute excess 90-day mortality attributable to acute kidney injury was estimated at 8.6 percentage points (95 CI, 2.6-17.6 percentage points). The population attributable risk (95 CI) of 90-day mortality associated with acute kidney injury was 19.6 (10.3-34.1 ). Conclusions: In general ICU patients, the absolute excess 90-day mortality statistically attributable to acute kidney injury is substantial (8.6 ), and the population attributable risk was nearly 20 . Our findings are useful in planning suitably powered future clinical trials to prevent and treat acute kidney injury in critically ill patients.