Rationale: Fish oil (FO), rich in omega-3 polyunsaturated fatty acids, exerts a range of anti-inflammatory actions that render it a potential therapeutic agent to treat Crohn's disease, a chronic inflammatory disease that primarily affects the bowel. Recent evidence suggests that a lack of effect in previous studies might be due to the fact that genetic background was not taken into account. For instance, a study in healthy subjects showed that production of the pro-inflammatory cytokine Tumor Necrosis Factor-alpha (TNF-α) following FO supplementation decreased in individuals within the highest tertile of pre-supplementational TNF-α production, remained unaltered in the middle tertile, and increased in the lowest tertile of pre-supplementational TNF-α production. TNF-α plays a pivotal role in the pathogenesis of Crohn's disease, hence the treatment with anti-TNF-α agents. Based on these notions, and because FO supplementation via the enteral route is strongly dose limited due to fat-induced side effects such as diarrhea, we hypothesize that parenteral FO supplementation might be beneficial in those patients with Crohn's disease with a high inherent TNF-α production.