BACKGROUND: Neurological deterioration (ND) is common, with nearly one-half of ND patients deteriorating within the first 24 to 48 hours of stroke. The timing of ND with respect to ND etiology and reversibility has not been investigated. METHODS: At our center, we define ND as an increase of 2 or more points in the National Institutes of Health Stroke Scale (NIHSS) score within 24 hours and categorize etiologies of ND according to clinical reversibility. ND etiologies were considered non-reversible if such causes may have produced or extended any areas of ischemic neurologic injury due to temporary or permanent impairment in cerebral perfusion. RESULTS: Seventy-one of 350 ischemic stroke patients experienced ND. Over half (54.9%) of the patients who experienced ND did so within the 48 hours of last seen normal. The median time to ND for non-reversible causes was 1.5 days (IQR 0.9, 2.4 days) versus 2.6 days for reversible causes (IQR 1.4, 5.5 days, p=0.011). After adjusting for NIHSS and hematocrit on admission, the log-normal survival model demonstrated that for each 1-year increase in a patient's age, we expect a 3.9% shorter time to ND (p=0.0257). In addition, adjusting for age and hematocrit on admission, we found that that for each 1-point increase in the admission NIHSS, we expect a 3.1% shorter time to ND (p=0.0034). CONCLUSIONS: We found that despite having similar stroke severity and age, patients with nonreversible causes of ND had significantly shorter median time to ND when compared to patients with reversible causes of ND.