OBJECTIVES: The aim of this study was to assess the treatment and rehospitalization for acute coronary syndrome (ACS) patients with dementia. METHODS: This is a nationwide population-based retrospective cohort study using administrative claims data National Health Insurance Research Database in Taiwan. A total of 212,110 patients who have been hospitalized for ACS between 2006 and 2010 were enrolled. Patients were categorized into five groups based on the number of guideline recommended medications (aspirin, beta-blockers, ACEI/ARB, statins, clopidogrel) used in comparison to using aspirin alone group. Cox proportional hazards regressions were performed to estimate the risk of rehospitalization for ACS associated with the number of medications used among all patients and patients with dementia. RESULTS: The study identified 212,110 patients with ACS; the mean age was 66.0 ± 12.9 and 34.6% were female. A higher number of guideline recommended medications was found to be associated with lower risk of rehospitalization for ACS. However, a non-significant increasing risk of rehospitalization for ACS was observed among 6486 ACS patients with dementia when a higher number of medications was prescribed. From 2-medications used group to 5-medications used group among ACS patients with dementia, the adjusted hazard ratios are 0.85 (95% CI 0.66-1.10), 0.99 (95% CI 0.71-1.37), 1.26 (95% CI 0.86-1.87) and 1.62 (95% CI 0.96-2.72) respectively. CONCLUSIONS: Among all ACS patients, the risk of rehospitalization for ACS decreased with increasing number of guideline recommended medications. However, this finding was not observed among ACS patients with dementia. The outcomes of treatment among ACS patients with dementia are different from other ACS patients.