American College of Cardiology/American Heart Association guidelines recommend angiotensin-converting enzyme inhibitor (ACEI) or angiotensin receptor blocker (ARB) therapy following acute myocardial infarction (MI) or development of heart failure (HF). This study estimated the effects of initiating these therapies after hospitalization for MI or HF on subsequent 1-year rehospitalization rates for MI or HF. A retrospective multivariate analysis of medical claims for 14,327 patients receiving and 7905 not receiving an ACEI or ARB after discharge for MI or HF was conducted. Rehospitalization for MI or HF was lower for treated vs untreated patients (MI: odds ratio [OR]=0.53, P<.001; HF: OR=0.52, P<.001). Rehospitalization was lower in treated patients with high medication compliance (medication possession ratio [MPR]>80%) and medium compliance (MPR 40%-79%) vs patients with low compliance (ORs for MI: high=0.54, medium=0.69; P<.001); ORs for HF: high=0.38, medium=0.62; P<.001). In conclusion, ACEI or ARB therapy initiated after hospitalization for MI or HF reduced risk of rehospitalization, and greater risk reduction was achieved with higher medication compliance.