ObjectiveTo improve epidemiologic knowledge of neurologic deterioration (ND) in patients with acute ischemic stroke (AIS).MethodsIn this prospective observational study, we captured ND prospectively in 29,446 patients with AIS admitted to 15 hospitals in Korea within 7 days of stroke onset. ND was defined as an increase in NIH Stroke Scale (NIHSS) score ≥2 (total), or ≥1 (motor or consciousness), or any new neurologic symptoms. Change in incidence rate after stroke onset, causes, factors associated with ND, modified Rankin Scale (mRS) score at 3 months and 1 year, and a composite of stroke, myocardial infarction, and all-cause death at 1 year were assessed.ResultsND occurred in 4,299 (14.6%) patients. The highest rate, 6.95 per 1,000 person-hours incidence, was within the first 6 hours, which decreased to 2.09 within 24–48 hours, and 0.66 within 72–96 hours after stroke onset. Old age, female sex, diabetes, early arrival, large artery atherosclerosis as a stroke subtype, high NIHSS scores, glucose level, systolic blood pressure, leukocytosis at admission, recanalization therapy, TIA without a relevant lesion, and steno-occlusion of relevant arteries were associated with ND. The causes were stroke progression (71.8%) followed by recurrence (8.5%). Adjusted relative risks (95% CI) for poor outcome (mRS 3–6) at 3 months and 1 year were 1.75 (1.70–1.80) and 1.70 (1.65–1.75), respectively. The adjusted hazard ratio (95% CI) for the composite event was 1.59 (1.45–1.74).ConclusionsND should be taken into consideration as a factor that may influence the outcome in acute ischemic stroke.