Objectives: To estimate the incidence of hospitalizations caused by a healthcare-associated adverse event (HAAE) in medical and surgical wards of public and private healthcare facilities. Method: A prospective assessment of HAE was conducted by external investigators—nurses and physicians—in collaboration with the healthcare team. The sample was randomized, stratified, and selected using three-stage cluster sampling, comprising all patients admitted over a seven-day period in each care unit. The primary outcome was the proportion of patients hospitalized due to an AHI. Results: A total of 4,839 patients were admitted to 292 care units in 71 healthcare facilities. Nearly 4 out of every 100 admissions were due to an adverse event (95% CI [3.3–4.6]), and 47% of these admissions were preventable. Invasive procedures, medical products, and infections accounted for 50%, 49%, and 22% of adverse events, respectively. Medication-related HAIs were the most preventable. HAIs primarily affected vulnerable patients. For 36% of the 195 HAIs, a care failure was identified. Discussion-Conclusion - Each year, 125,000 to 205,000 hospital stays caused by adverse events could be prevented. These stays represent only a portion of iatrogenic events, approximately 40% of those observed in hospital settings. (R.A.)