Background: Post-discharge surveillance for 30 days is needed to determine the true incidence of surgical site infection (SSI). This study was undertaken to determine the incidence of, and risk factors for, SSI after hernia repair. Methods: A total of 3150 patients who had undergone groin hernia repair in 32 Scottish hospitals were telephoned 10, 20 and 30 days after operation to screen for SSI. Patients who believed the wound to be infected were seen by a healthcare worker to confirm the diagnosis. Details of operations and risk factors were obtained by case-note review. Results: One hundred and four patients (3·3 per cent) declined to give a contact telephone number, leaving 3046 patients who agreed to take part in the study. Some 108 patients (3·4 per cent) could not be contacted at any point, giving a response rate of 93·3 per cent. Complete data were available for 2665 patients (87·5 per cent); 140 (5·3 per cent) developed SSI and 57 (2·1 per cent) thought the wound infected but this was not confirmed by the healthcare worker. Patients given a prophylactic antibiotic had a lower incidence of SSI (P = 0·002), but neither increase in the American Society of Anesthesiologists grade of fitness for operation nor prolonged duration of operation was a significant risk factor for infection. Conclusion: SSI after hernia repair is common and large clinical trials are required to determine whether the use of prophylactic antibiotics reduces the incidence of infection.