Cancer patients face an increased risk of arterial thromboembolism; however, it is uncertain when this excess risk begins. This study evaluated the risk of arterial thromboembolism before cancer diagnosis. Using the population-based Surveillance Epidemiology and End Results-Medicare linked dataset, we identified 374 331 patients ≥67 years of age with a new primary diagnosis of breast, lung, prostate, colorectal, bladder, uterine, pancreatic, gastric cancer, or non-Hodgkin lymphoma from 2005 through 2013. Cancer patients were individually matched by demographics and comorbidities to Medicare beneficiaries without cancer, who served as controls. Validated diagnosis codes were used to identify arterial thromboembolic events, defined as a composite of myocardial infarction or ischemic stroke. The Mantel-Haenszel estimator was used to compare risks of arterial thromboembolic events between cancer and noncancer groups during 30-day periods in the 360 days before date of cancer diagnosis. From 360 to 151 days before cancer diagnosis, the 30-day interval risks of arterial thromboembolic events were similar between cancer patients and matched controls. From 150 to 1 day before cancer diagnosis, the interval 30-day risks of arterial thromboembolic events were higher in cancer patients vs matched controls, progressively increasing as the cancer diagnosis date approached and peaking during the 30 days immediately before cancer diagnosis, when 2313 (0.62%) cancer patients were diagnosed with an arterial thromboembolic event vs 413 (0.11%) controls (odds ratio, 5.63; 95% confidence interval, 5.07-6.25). In conclusion, the risk of arterial thromboembolic events begins to increase 150 days before the date of cancer diagnosis in older persons and peaks in the 30 days before. •Among 748 662 Medicare beneficiaries, the risk of arterial thromboembolic events was increased 69% in the year before cancer diagnosis.•The increased risk of arterial thromboembolic events began 5 months before cancer was officially diagnosed and peaked in the month prior. Display omitted