A growing number of surveillance studies have highlighted concerns with relying only on data from inpatients. Without post-discharge surveillance (PDS) data, the rate and burden of surgical site infections (SSIs) are underestimated. PDS data for colorectal surgery in the UK remains to be published. This is an important specialty to study since it is considered to have the highest SSI rate and is among the most expensive to treat. This study of colorectal SSI used a 30 day surveillance programme with telephone interviews and home visits. Each additional healthcare resource used by patients with SSI was documented and costed. Of the 105 patients who met the inclusion criteria and completed the 30 day follow-up, 29 (27%) developed SSI, of which 12 were diagnosed after discharge. The mean number of days to presentation of SSI was 13. Multivariable logistic analysis identified body mass index as the only significant risk factor. The additional cost of treating each infected patient was pound sterling 10,523, although 15% of these additional costs were met by primary care. The 5 month surveillance programme cost pound sterling 5,200 to run. An analysis of the surveillance nurse's workload showed that the nurse could be replaced by a healthcare assistant. PDS to detect SSI after colorectal surgery is necessary to provide complete data with accurate additional costs.