Background: Many clinical investigations depend on participant self-report as a principal method of identifying health care events. If self-report is used as the trigger to collect and adjudicate medical records, any event that is not reported by the patient will be missed by the investigators, reducing the power of the study and misrepresenting the risk of its participants. We sought to determine the rates and predictors of underreporting hospitalization events during the follow-up period of a prospective study of patients hospitalized with an acute myocardial infarction. Methods and Results: The TRIUMPH (Translational Research Investigating Underlying Disparities in Acute Myocardial Infarction Patients’ Health Status) registry, a longitudinal multicenter cohort study of people with acute myocardial infarction in the United States, queried patients for hospitalization events during interviews at 1, 6, and 12 months. To validate these self-reports, medical records for all events at every hospital where the patient reported receiving care were acquired for adjudication, not just those for the reported events. Of the 4340 participants in TRIUMPH, 1209 (28%) reported at least one hospitalization. After medical records abstraction and adjudication, we identified 1086 hospitalizations from 639 participants (60.2±12 years of age, 38.2% women). Of these hospitalizations, 346 (31.9%) were underreported by the participants. Rates of underreporting ranged from 22.5% to 55.6% based on different patient characteristics. The odds of underreporting were highest for those not currently working (adjusted odds ratio, 1.66 [95% CI, 1.04–2.63]), lowest for those married (adjusted odds ratio, 0.50 [95% CI, 0.33–0.76]), and increased the longer the elapsed time between the admission and the patient’s follow-up interview (adjusted odds ratio per month, 1.16 [95% CI, 1.08–1.24]). There was a substantial within-individual variation on the accuracy of reporting. Conclusions: Hospitalizations after acute myocardial infarction are commonly underreported in interviews and should not be used alone to determine event rates in clinical studies.