The inflammatory response to surgery is essential for healing and recovery, however hyperinflammation and altered immune competence increases the risk of inflammation mediated complications. We hypothesised that, in some patients, a state of postoperative systemic inflammatory dysregulation (PSID) exists increasing the risk or infection and sepsis, and that this will be associated with, and potentially predicted by, altered patterns of genome-wide peripheral blood mononuclear cell differential DNA methylation and gene expression. In this study we utilised phenotypic extremes of postoperative plasma C-reactive protein levels to define PSID, which manifested clinically in significantly higher rates of sepsis, complications and longer hospital stays following major abdominal surgery. We identified altered DNA methylation and differential gene expression in specific immune and metabolic pathways during PSID. Our findings suggest that dysregulation results in, or from, dramatic changes in differential DNA methylation and highlights potential targets for early detection and treatment. The combination of altered DNA methylation and gene expression confirms that dysregulation is mediated at multiple levels within specific gene sets and hence, non-specific anti-inflammatory treatments such as corticosteroids alone are unlikely to represent an effective therapeutic strategy. Overall design: In this study we have matched RNA-seq expression and EPIC methylation array data from patients before and after major abdominal surgery. There are two patient groups, those with low CRP after surgery (n= 25) and those with elevated CRP after surgery (n=21). The purpose is to understand the gene regulatory events that occur in patients with high CRP levels, as these patients are at a higher risk of adverse outcomes after surgery.