Objectives Administration of eicosapentaenoic acid (EPA) and docosahexanoic acid (DHA), omega-3 fatty acids in fish oil, has been associated with improved patient outcomes in acute lung injury (ALI) when studied in a commercial enteral formula. However, fish oil has not been tested independently in ALI. We therefore sought to determine if enteral fish oil alone would reduce pulmonary and systemic inflammation in patients with ALI. Design Phase II randomized controlled trial. Setting Four North American medical centers. Patients Mechanically ventilated patients with ALI ≥ 18 years of age. Interventions Subjects were randomized to receive enteral fish oil (9.75g EPA and 6.75g DHA daily) or saline placebo for up to 14 days. Measurements and Main Results Bronchoalveolar lavage fluid (BALF) and blood were collected at baseline (day 0), day 4±1, and day 8±1. The primary endpoint was BALF interleukin (IL)-8 levels. Forty-one participants received fish oil and 49 received placebo. Enteral fish oil administration was associated with increased serum EPA concentration (p<0.0001). However, there was no significant difference in the change in BALF IL-8 from baseline to day 4 (p=0.37) or day 8 (p=0.55) between treatment arms. There were no appreciable improvements in other BALF or plasma biomarkers in the fish oil group compared to the control group. Similarly, organ failure score, ventilator-free days, ICU-free days, and 60-day mortality did not differ between the groups. Conclusions Fish oil did not reduce biomarkers of pulmonary or systemic inflammation in patients with ALI, and the results do not support the conduct of a larger clinical trial in this population with this agent. This experimental approach is feasible for proof of concept studies evaluating new treatments for ALI.