We hypothesized that left cerebral hemisphere infarction is more frequent than right hemisphere infarction among young adults. Furthermore, we aimed to evaluate possible causes for this asymmetry. Cases were found by computer search from hospital registries at each of the 5 acute-care hospitals in a well-defined population in Hordaland County, western Norway. Important parameters used in the analysis were derived from the patient records. A total of 98 patients had left hemisphere infarctions, and 70 patients had right hemisphere infarctions (P = .037). This difference was due mostly to the higher incidence of infarction in the left middle cerebral artery (MCA) territory compared with the right MCA territory among the male subjects (P = .016). Lacunar infarction was equally distributed, whereas nonlacunar infarction was more frequent in the left MCA territory among the men (P = .016). A high frequency of left MCA infarctions may be associated with more frequent atherosclerosis in the left carotid artery, lateralization of cortical functions, or both in young adults.