Although surgical site infections (SSIs) occurring after hospital discharge cause substantial morbidity, their epidemiology is not well understood, and methods for routine postdischarge surveillance have not been validated. Inpatient and outpatient surveillance followed 5572 nonobstetric procedures among members of a health maintenance organization with extensive automated records. Records were screened for coded diagnoses, tests, and prescriptions and, if positive, were reviewed by reading full text. Questionnaires regarding the occurrence of an SSI were sent to the same patients and their surgeons. One hundred thirty-two SSIs were documented, of which 84% occurred after hospital discharge and 63% were managed outside the surgical facility. Postdischarge SSIs led to an average of 4.6 additional ambulatory encounters. Patient and surgeon questionnaires had a sensitivity of 28% and 15%, respectively. These data suggest that most SSIs occur after discharge and are not detectable by conventional surveillance. Nonetheless, they cause substantial resource utilization.