Background: The course of serum cytokine levels in patients with postoperative systemic inflammatory response syndrome (SIRS) after major abdominal surgery remains currently unclear. Methods: Blood was sampled pre- and post-operatively and on days 1 and 2 in 40 patients undergoing major abdominal surgery. Concentrations of tumour necrosis factor-alpha (TNFα), interleukin (IL) -6, IL-8, and IL-10 were measured by the LINCOplex assay; those of soluble triggering receptor expressed on myeloid cells-1 (sTREM-1) by an enzyme immunoassay. Results: Compared to their pre-operative values, sTREM-1 was elevated on day 2; TNFα on day 1; IL-6 and IL-10 post-operatively and on days 1 and 2; and IL-8 post-operatively and on day 1. The duration of operation correlated with TNFα and IL-10 at all sampling times, and with IL-6 post-operatively. There were no differences in cytokine concentrations between patients who exhibited post-operative complications and those who did not. IL-10/TNFα below 30 was found in all patients with complications (100%) and in 20 patients without complications (64.5%, p: 0.043). Conclusions: SIRS following major surgery is characterised by complex alterations in cytokine concentrations. The balance between TNFα and IL-10 seems to determine the occurrence of post-operative complications.