BACKGROUND: It has previously been shown that both very long chain omega-3 polyunsaturated fatty acids (n-3 PUFA) and a Mediterranean-like diet (Md), are able to reduce the risk of cardiovascular (CV) mortality and morbidity. The exact mechanisms behind this effect are yet to be established. To date, there exist no data on the effect of n-3 PUFA supplementation and Md on components of the interleukin-6 (IL-6) trans-signalling (ts) system that plays a central role in the family of pro-atherosclerotic inflammatory markers. METHODS: A total of 563 men were included in the DOIT study, a randomised factorial-designed trial comparing the effect of 36 months of dietary counseling, n-3 PUFA supplementation (2.4 g/d), or both on different circulating biomarkers of atherosclerosis in elderly high-risk men. We used commercially available ELISA methods to analyse circulating levels of soluble glycoprotein 130 (sGP130), soluble IL-6 receptor (sIL-6r), and IL-6. RESULTS: There was no significant effect of either of the intervention principles on circulating levels of sGP130 or sIL-6r. We have shown previously that there is no effect on IL-6 concentrations either. CONCLUSIONS: This is the largest trial analysing possible effects of Md or n-3 PUFA supplementation on the IL-6ts system. Although the reduction of CV risk through dietary intervention or n-3 PUFA supplementation has previously been linked to anti-inflammatory effects, we could not find an effect of these interventions on the IL-6ts system. This indicates that the beneficial effects of Md or n-3 PUFA observed in previous studies seem to be independent of the IL-6ts system.