It is common to find computed tomography scan evidence of prior stroke without a history of such an event. The frequency, risk factors for, and relevance of silent strokes are unknown. The Framingham cohort of 5,184 men and women aged 30-62 years and free of stroke at entry to the study have been followed with periodic examinations since 1950. We studied the silent strokes found on computed tomography scan of all initial strokes that occurred between January 1, 1979, and July 31, 1987. During these 8 1/2 years, 164 initial strokes occurred; 124 had computed tomography scans performed. There were 13 (10%) with silent stroke, 71 had abnormalities related to their presenting acute stroke, and 40 had normal computed tomography scans. There were 15 silent lesions; eight were lacunar infarcts in the basal ganglia-internal capsule area, seven were small cortical infarcts. Glucose intolerance was the sole risk factor that occurred significantly more frequently (11 of 13) in the group with silent lesions (p less than 0.04) than in the group with computed tomography evidence of acute stroke. Silent stroke is not rare; it was present in at least 10% of acute initial stroke patients arising in a general population. The relation of these silent lesions to the development of "vascular" dementia and poststroke disability deserves further study.