<b>Justification: </b>Acute myocardial infarction is a major public health problem. In Costa Rica, it is necessary to record it accurately in order to handle this problem efficiently. <b>Materials and methods: </b>Descriptive and observational study. Data from patients which were discharged from the Mexico Hospital and diagnosed with acute coronary syndrome, acute myocardial infarction, unstable angina and ischemic heart disease from August 2005 to July 2006 was compiled. The records of the MH- Biostatistics office, as well as those from a series of Units (Coronary, Intensive Care, Hemodynamics and Echocardiograms)- were analyzed. <b>Results:</b> The Mexico Hospital reports 110 patients diagnosed with acute myocardial infarction. This number increased to 172 when patients discharged as cases of ACS, unstable angina or ischemic heart disease were reported also as cases of acute myocardial infarction. The final sample analyzed was 138 patients when some patients were excluded due to incomplete data. This meant that underreporting was at least 36 %. Men represented 78.1 % of the sample; the average age for both sexes was 65.2 years. Troponin was not measured in 49.3% of the patients and a quarter of them were not assessed by a cardiologist. Out of the total of patients discharged with other diagnoses, 20.3% also had AMI. <b>Conclusions: </b>Acute myocardial infarction is under reported in the Mexico Hospital, and in the central offices of the Ministry of Health and of the Costa Rican Social Security System. Findings could be similar in other Costa Rican hospitals.

acute myocardial infarction; acute coronary syndrome; underreport.