Objectives: Our goal was to assess the prevalence of complicated American Heart Association (AHA) lesion type VI plaques in the carotid arteries of patients with cryptogenic stroke. Background: In up to 40% of ischemic stroke patients, no definite cause can be established despite extensive workup (i.e., cryptogenic stroke). To test the hypothesis if nonstenosing complicated carotid plaques may be the underlying etiology in some of these patients, we used high-resolution black-blood carotid magnetic resonance imaging (MRI), which can quantitatively assess plaque composition and morphology with good correlation to histopathology. Specifically, we focused on AHA type VI plaques, which are characterized by hemorrhage, thrombus, or fibrous cap rupture. Methods: Thirty-two consecutive patients (22 male; mean age 71.7 ± 11.9 years) with cryptogenic stroke and nonstenosing (<50%) eccentric carotid plaques were recruited from a single stroke unit. All patients underwent extensive clinical workup (brain MRI, duplex sonography, electrocardiography and Holter monitoring, transthoracic and transesophageal echocardiography, and laboratory investigations) to exclude other causes of stroke. All patients received a black-blood carotid MRI at 3-T with fat-saturated pre- and post-contrast T-1-, proton density-, and T-2-weighted and time-of-flight images using surface coils and parallel imaging techniques. Prevalence of AHA type VI plaque was determined in both carotid arteries on the basis of previously published MRI criteria. Results: AHA type VI plaques were found in 12 of 32 arteries (37.5%) ipsilateral to the stroke, whereas there were no AHA type VI plaques contralateral to the stroke (p = 0.001). The most common diagnostic feature of AHA type VI plaques was intraplaque hemorrhage (75%), followed by fibrous plaque rupture (50%) and luminal thrombus (33%). Conclusions: This pilot study suggests that arterio-arterial embolism from complicated, nonstenosing carotid atherosclerotic plaques may play a role in a subgroup of patients previously diagnosed with cryptogenic stroke. To further evaluate the significance of AHA type VI plaques in cryptogenic stroke, future studies will have to analyze both clinical and imaging follow-up data, including event rates for secondary strokes.