OBJECTIVES To report on the results of the Swiss national surgical site infection (SSI) surveillance program, including temporal trends, and to describe methodological characteristics that may influence SSI rates DESIGN Countrywide survey of SSI over a 4-year period. Analysis of prospectively collected data including patient and procedure characteristics as well as aggregated SSI rates stratified by risk categories, type of SSI, and time of diagnosis. Temporal trends were analyzed using stepwise multivariate logistic regression models with adjustment of the effect of the duration of participation in the surveillance program for confounding factors. SETTING The study included 164 Swiss public and private hospitals with surgical activities. RESULTS From October 2011 to September 2015, a total of 187,501 operations performed in this setting were included. Cumulative SSI rates varied from 0.9% for knee arthroplasty to 14.4% for colon surgery. Postdischarge follow-up was completed in >90% of patients at 1 month for surgeries without an implant and in >80% of patients at 12 months for surgeries with an implant. High rates of SSIs were detected postdischarge, from 20.7% in colon surgeries to 93.3% in knee arthroplasties. Overall, the impact of the duration of surveillance was significantly and independently associated with a decrease in SSI rates in herniorraphies and C-sections but not for the other procedures. Nevertheless, some hospitals observed significant decreases in their rates for various procedures. CONCLUSIONS Intensive post-discharge surveillance may explain high SSI rates and cause artificial differences between programs. Surveillance per se, without structured and mandatory quality improvement efforts, may not produce the expected decrease in SSI rates. Infect Control Hosp Epidemiol 2017;38:697-704.