Epidemiological studies of acute kidney injury (AKI) and acute-on-chronic renal failure (ACRF) are surprisingly sparse and confounded by differences in definition. Reported incidences vary, with few studies being population-based. Given this and our aging population, the incidence of AKI may be much higher than currently thought. We tested the hypothesis that the incidence is higher by including all patients with AKI (in a geographical population base of 523,390) regardless of whether they required renal replacement therapy irrespective of the hospital setting in which they were treated. We also tested the hypothesis that the Risk, Injury, Failure, Loss, and End-Stage Kidney (RIFLE) classification predicts outcomes. We identified all patients with serum creatinine concentrations > or =150 micromol/L (male) or > or =130 micromol/L (female) over a 6-mo period in 2003. Clinical outcomes were obtained from each patient's case records. The incidences of AKI and ACRF were 1811 and 336 per million population, respectively. Median age was 76 yr for AKI and 80.5 yr for ACRF. Sepsis was a precipitating factor in 47% of patients. The RIFLE classification was useful for predicting full recovery of renal function (P < 0.001), renal replacement therapy requirement (P < 0.001), length of hospital stay [excluding those who died during admission (P < 0.001)], and in-hospital mortality (P = 0.035). RIFLE did not predict mortality at 90 d or 6 mo. Thus the incidence of AKI is much higher than previously thought, with implications for service planning and providing information to colleagues about methods to prevent deterioration of renal function. The RIFLE classification is useful for identifying patients at greatest risk of adverse short-term outcomes.