Previous brain infarctions seen on CT are common in the absence of history of stroke. Eleven percent of patients (135/1,203) without stroke history had ischemic lesions on their first CT, unrelated to the presenting stroke. Stroke Data Bank files were reviewed to determine whether the occurrence, location, and CT characteristics of those lesions are associated with their undetected status. Two distinct patterns were seen: (1) Small lesions of 1 cm or less were left hemisphere predominant and involved the deep structures of the brain; the majority of these were clinically silent. (2) Lesions greater than 1 cm occurred more frequently in the right hemisphere and were mostly superficial; these infarcts produced signs unrecognized by the patient or family. Risk factors for silent stroke were similar to those for stroke in general. Frequencies of in-hospital complications and 30-day fatality rates from the presenting stroke were not influenced by the existence of a prior silent stroke.