The purpose of this study is to assess whether postoperative complications can be ascertained using administrative data. We randomly sampled 991 adults who underwent elective open diskectomies at 30 nonfederal acute care hospitals in California. Postoperative complications were specified by reviewing medical literature and by consulting clinical experts. We compared hospital-reported ICD-9-CM data and independently recoded ICD-9-CM data with complications abstracted by clinicians using detailed criteria. Recoded ICD-9-CM data were more likely than hospital-reported ICD-9-CM data to capture true complications, when they occurred, but they also mislabeled more patients who never experienced clinically significant complications. This finding was most evident for mild or ambiguous complications, such as atelectasis, posthemorrhagic anemia, and hypotension. Overall, recoded ICD-9-CM data captured 47% and 56% of all mild and severe complications, respectively, whereas hospital-reported ICD-9-CM data captured only 37% and 44%, respectively, of all mild and severe complications. These findings raise questions about the validity of using administrative data to ascertain postoperative complications, even if coders are carefully hired, trained, and supervised. ICD-9-CM complication codes are more promising as a tool to help providers identify their own adverse outcomes than as a tool for comparing performance.