Abstract Background: Hospital accreditation has as goal the standardization of patient care, aiming quality improvement. On 2015, a cardiology reference hospital was evaluated and got level 3 from ONA in care given to Acute Coronary Syndrome (ACS) patients. Objetive: To compare length of stay (LOS) at Coronary Care Unit (CCU) and the total LOS at the hospital of ACS patients before and after ONA 3 accreditation. Other clinical outcomes were also analyzed. Methods: Systematic and prospective registry of admitted ACS patients at CCU, whose population was divided into pre-accreditation (period 1) and post-accreditation (period 2). Descriptive analysis was performed. For statistical analysis the Mann-Whitney test, chi-squared, Fisher’s exact test and Multiple Linear Regression were performed. P value was considered statistically significant when < 0,05. Results: 372 patients were admitted with ACS, 186 in period 1, of which 47 (25,3%) with ST segment Elevation Myocardial Infarction (STEMI), and 186 in period 2, of which 70 (37,6%) with STEMI. The mean age was 65,9 years (± 12,2). About the CCU LOS, there was a reduction from 3 (IQR: 2-4) to 2,5 days (IQR: 2-4; p value = 0,088). Regarding the hospital LOS, there was also a reduction from 8 (IQR: 5-12,25) to 6 days (IQR:4-11; p value = 0,004). Analyzing the type of ACS, there was a significant reduction only at the hospital LOS in non-STEMI patients: 8 to 6 days (p value = 0,001). Other hospitalization length of stay and clinical outcomes did not present a significant reduction in the comparison. Conclusion: After the ONA 3 accreditation, there was a reduction of hospital LOS. There were no significant differences in the other outcomes analyzed.