In a prospective study of 128 patients with acute ischemic stroke less than 24 h from onset, we explored new predictive factors for early neurological deterioration (a decrease in Canadian Stroke Scale score during the first 48 h of ≧ 1 point). A computed tomographic scan was performed before inclusion and repeated between days 4 and 7. Independent factors predictive of progression were identified by logistic regression analysis. There was worsening during the first 48 h in 33.6% of the patients. Body temperature (p < 0.0001) and plasma fibrinogen (p = 0.003) were independently related to progressing stroke. For each 1 °C increase in body temperature, the relative risk of progression rose by 9.2 (95% CI, 4–21). The percentage of correct classifications was 83%. Thus, hyperthermia and plasma fibrinogen levels within the first 24 h of acute ischemic stroke are related to early neurological worse