Complicated intra-abdominal infection (CABI) extends beyond the hollow viscus of origin into the peritoneal space and is associated with either abscess formation or peritonitis [Solomkin 2010]. CABIs are heterogeneous in aetiology and include spontaneous infections arising from a perforated viscus, including the stomach, biliary tree, colon, appendix and reproductive organs. In addition there are post-operative complications such as peritonitis, abscess, and failure of surgical anastomoses. CABIs are also heterogeneous in their characteristics. A recent review of organ space Surgical Site Infections (SSI-Os) after colorectal surgery demonstrated that infections were varied with regard their location, as well as size and number and nature of collections, and presence of an ongoing source e.g. anastomotic breakdown (Rothwell 2016). Despite the varied origin of these infections, there are similar management strategies which centre on the source control, e.g. drainage of intra-abdominal fluid collections, and administration of antibiotic therapy. CABIs are a common in-hospital clinical challenge, in part due to the varied pathology they are caused by, and are associated with significant morbidity and mortality (DeFrances 2005, Brun-Buisson 1995). Despite this burden of disease, there is little clinical evidence with which to base treatment on, even before taking account of their heterogeneous nature. One study of CABI following source control procedures evaluated a short course of antibiotics (4 days) compared to a course of up to 2 weeks after clinical and biochemical improvement (up to 10 days). Whilst there was little difference in outcomes, both groups had a high relapse rate of approximately 15%. In addition, it is common for source control procedures not to be completed in routine clinical practice, being completed in only 17% of Leeds patients with post-operative CABI (Rothwell 2016). In this Leeds data of post-operative CABIs, where source control rates are low relapse rates were 40% (Rothwell 2016). For CABI infections, standard UK management is variable and involves between 4 and 42 days' antibiotics. This indicates an unacceptably high relapse rate in patients treated for CABI where source control is and is not undertaken. We therefore propose to investigate if long course antibiotic therapy (28 days) is clinically more effective than short course antibiotics (4 days) in preventing relapses of CABI. This study will include patients who have and have not undergone source control procedures which will be completed as per standard practice.