Background: Acute lung injury (ALI) has been primarily defined in patients who require positive pressure ventilation. As a result, the clinical characteristics of patients with early ALI (EALI) prior to the need for mechanical ventilation have not been well characterized. Early identification of patients with ALI and the impending need for positive pressure ventilation could define a study population for trials of novel therapies. Methods: We analyzed clinical data from 93 patients at 12, 24, and 48 hours prior to the standard diagnosis of ALI. The time of ALI diagnosis was defined when patients were mechanically ventilated and met the 1994 American-European Consensus Conference diagnostic criteria for ALI. Results: The majority of patients with ALI presented to the hospital more than 24 hours prior to developing ALI. Specifically, 73% presented more than 12 hours prior to diagnosis, and 57% presented more than 24 hours prior to diagnosis. Of patients hospitalized for at least 12 hours prior to ALI diagnosis, 94% had either bilateral infiltrates on chest radiograph, tachypnea, or an oxygen requirement greater than 2 L/min; 79% and 48% had 2 and 3 of these abnormalities, respectively. Conclusion: The majority of hospitalized patients who are destined to develop ALI demonstrate tachypnea, increased oxygen requirements, and/or bilateral infiltrates on chest radiograph more than 12 hours prior to meeting criteria for diagnosis. Some patients with EALI may be identified prior to meeting diagnostic criteria during a potential therapeutic window.