OBJECTIVES:  To examine the relationship between adherence to both statins and ACEIs/ARBs/beta-blockers and coronary artery disease (CAD)-related hospitalizations. METHODS: A retrospective observational longitudinal study was conducted using SEER-Medicare data. Elderly Medicare fee-for-service beneficiaries with pre-existing CAD and incident breast, colorectal, or prostate cancer (N = 12,096) were observed for a period of 12 months before and 12 months after cancer diagnosis. Hospitalizations measured every 120-days were categorized into three groups: (1) any CAD-related hospitalization; (2) other hospitalizations; or (3) no hospitalization. Medication adherence was categorized into five mutually exclusive groups: (1) adherent to both statins and ACEIs/ARBs/beta-blockers (reference group); (2) not adherent to both statins and ACEIs/ARBs/beta-blockers; (3) adherent to either statins or ACEIs/ARBs/beta-blockers; (4) use of one medication class and adherent to that class; or (5) use of one medication class and not adherent to that medication class. The unadjusted and adjusted relationship between medication adherence and hospitalization was analyzed using repeated measures multinomial logistic regressions. Inverse probability treatment weights were used to control for observed group differences among medication adherence categories. RESULTS:  Adherence to both statins and ACEIs/ARBs/beta-blockers was estimated at 31.2% during the 120-day period immediately after cancer diagnosis; 13.7% were not adherent to both medication classes during the same period; 27.4% had CAD-related hospitalizations immediately after cancer diagnosis and this percentage declined to 10.6% during the last four months of the post-cancer period. In the adjusted analyses, those not adherent to both statins and ACEIs/ARBs/beta-blockers were more likely to have CAD-related hospitalization compared to those who were adherent to both medication classes [AOR = 1.82; 95% CI = 1.72, 1.92; P < 0.0001]. CONCLUSIONS:  Given the complexity of interaction between CAD and cancer, it is important to routinely monitor medication adherence in general clinical practice and provide linkages to support services that can increase medication adherence.