The aim of this study was to identify the frequency and possible pathogenic mechanisms of early neurological deterioration in patients with acute small non-lacunar infarction. We studied 46 patients (35 men, 11 women; age, 70.3+/-10.4 years) with acute small non-lacunar infarction. Small non-lacunar infarction was diagnosed using diffusion-weighted magnetic resonance imaging (DWI) as being <15 mm in diameter and located in the cortex and centrum ovale in the middle cerebral artery territory. The patients were divided into two groups; Group D (n=6) had neurological deterioration within 7 days after symptom onset, while Group N (n=40) did not have any neurological deterioration. In Group D, the interval from symptom onset to clinical deterioration was 3.3+/-1.5 days (range 2-6 days). Blood pressure on admission was higher in Group D than in Group N (p<0.05). In Group D, four of these five patients with follow-up DWI had new acute small ischemic lesions in addition to the initial lesions, indicating recurrent attacks of brain infarction. Neurological deterioration occurred within 7 days after symptom onset in 13% of patients. Neurological deterioration was frequently caused by recurrent infarction detected by DWI.