We used the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) primary diagnosis codes 433, 434, 436, 437.0, and 437.1 to identify the patients admitted with ischemic stroke. Patients who underwent thrombolytic treatment were identified by procedure code 99.10. In 1998, a new procedure code, 99.10, was designated for injection or infusion of thrombolytic agents, permitting estimation of national and statewide use.12,13 Patients with dissection were identified using ICD-9-CM codes 443.2, 443.21, and 443.24 as any of the 15 secondary discharge diagnoses in patient records. Cardiovascular risk factor information was obtained from the Agency for Healthcare Research and Quality comorbidity data collected for each patient. The ICD-9-CM secondary diagnosis codes were used to identify those with stroke-associated complications such as pneumonia (486, 481, 482.8, and 482.3), urinary tract infection (599.0 and 590.9), sepsis (995.91, 996.64, 038, 995.92, and 999.3), deep venous thrombosis (451.1, 451.2, 451.81, 451.9, 453.1, 453.2, 453.8, and 453.9), pulmonary embolism (415.1), intracerebral or subarachnoid hemorrhage (430, 431, and 432), and acute myocardial infarction (410). We also used ICD-9-CM procedure codes to estimate the percentage of stroke patients who underwent stroke-related procedures such as cerebral angiography (88.41), carotid angioplasty/stent placement (00.63/00.64), intracranial angioplasty/stent placement (00.62/00.65), gastrostomy (43.11-43.19), tracheostomy (31.10, 31.20, 31.21, or 31.29), and carotid endarterectomy (38.12) and those who had postprocedure stroke (997.02). Mechanical thrombectomy and/or intra-arterial thrombolytic administration was identified using diagnosis-related group codes 23, 24, or 543 or ICD-9-CM procedure codes 00.41 to 00.43 or 39.74. Posterior circulation dissections were identified by ICD-9-CM codes 433.01 and 433.2 for stenosis/occlusion involving the basilar or vertebral arteries. All other dissections were classified as anterior circulation dissections. Indeterminate location was used if none of the arterial location codes were used.