In various diseases n-3 fatty acids exert anti-inflammatory properties. These effects seem to be related to the uptake and incorporation of eicosapentaenoic acid (EPA) into the cellular substrate pool after dietary intake of EPA, which is contained in fish oils (FO). In the state of inflammation EPA is released to compete with arachidonic acid (AA) for metabolism at the cyclo-oxygenase and the 5-lipoxygenase level. The metabolites of EPA have less inflammatory and chemotactic potency than the substances derived from AA. In addition to positive effects, early studies pointed towards prolonged bleeding times after dietary intake of n-3 fatty acids. This study was undertaken to address the issue of potential coagulation disturbances associated with postoperative parenteral FO administration. This was a prospective, randomised, double blinded clinical trial, carried out in two operative intensive care units (13 and 16 beds) in a university hospital. Forty-four patients undergoing elective major abdominal surgery participated in the trial. Patients were randomly assigned to receive total parenteral nutrition (TPN) supplemented with either soybean oil (SO, Lipovenoess 10% PLR; 1.0 g/kgBW per day; n = 20) for five days or with a combination of FO and SO (FO, Omegaven; 0.2 g/kgBW per day plus SO, Lipovenoes 10% PLR; 0.8 g/kgBW per day, n = 24), respectively. Blood samples were taken preoperatively (day -1), prior to (day 1) during (days 2-5) and after TPN (day 6). The coagulation parameters thromboplastin time (Quick), activated partial thromboplastin time (aPTT), fibrinogen and antithrombin III were measured. To differentially assess activation levels of extrinsic and intrinsic coagulation pathway, factors VIIa and XIIa were quantified. Moreover platelet function was determined by resonance thrombography. Baseline values of coagulation and platelet function were comparable in both groups, but coagulation activity dropped after surgery. Over the observation period of 6 days, however, physiological levels were regained. No clinically significant differences were observed between the SO- and SO + FO- group. These findings suggest that infusion of fish oil in doses up to 0.2 g/kgBW per day is safe regarding coagulation and platelet function.