['The purpose of this study is to identify the predictors for prolonged mechanical ventilation (PMV) of more than 21 days among intensive care unit (ICU) patients.', 'A retrospective observational study was conducted in a respiratory ICU from December 2008 to November 2009. The outcome measurement was the occurrence of PMV. Acute kidney injury (AKI) was identified and defined as an increase in the serum creatinine level of 50% or greater from baseline.', 'Of 154 patients enrolled, 41 patients (26.6%) had PMV. Patients with PMV showed higher Acute Physiology and Chronic Health Evaluation II scores, lower serum albumin levels, and more AKI on mechanical ventilation (MV) initiation day compared with the non-PMV patients. Patients with PMV were significantly associated with longer MV duration before the day of readiness for weaning (DRW) and a higher rapid shallow breathing index on DRW. In a multivariate regression analysis, the independent risk factors for PMV were AKI on MV initiation day (odds ratio [OR], 5.630; 95% confidence interval [CI], 1.378-22.994; P = .016), longer MV duration before DRW (OR, 1.289; 95% CI, 1.158-1.435; P < .001), and higher rapid shallow breathing index on DRW (OR, 1.012; 95% CI, 1.003-1.021; P = .010).'] Does acute kidney injury on ventilator initiation day independently predict prolonged mechanical ventilation in intensive care unit patients?