Surgical site infection (SSI) is an important outcome indicator. It is estimated that 70% of post-operative infections present after discharge. A reliable post-discharge surveillance (PDS) method is yet to be described. The aim of this prospective cohort study was to assess the reliability of patient self-diagnosis. Telephone questionnaires were used following hip and knee prosthetic surgery. A trained validation nurse checked the wounds of all patients reporting problems and a sample of those who did not. 376 elective hip and knee arthroplasty procedures from 363 patients were included. In-patient infection rate was 3.1% (13 of 422 procedures) and post-discharge infection rate was 5.2% (22 of 422 procedures). Results suggest that patients can reliably self diagnose SSI. The sensitivity of the procedure (the probability that the telephone surveillance will detect an infection given that the patients has an infection) was 90.9%. The specificity (the probability that the telephone surveillance will report no infection given that no infection is present) was 76.6%. Hence telephone PDS of SSI is a valuable means of identifying accurate rates of hospital acquired infection following surgery. In this study population, 41% of infections were diagnosed post discharge, which is lower than has previously been estimated. PDS of SSI is necessary if accurate rates of hospital acquired infection following surgery are to be available.