Previous studies showed that elevated body temperature early after ischemic stroke is associated with severe neurological deficit and a poor outcome. The aim of this study was to analyse the prevalence and putative etiology of febrile body temperature (>/=38.0 degrees C) early after stroke and to investigate the association between body temperature, stroke severity and outcome. We investigated 119 consecutive patients who were admitted within 24 h after ischemic stroke. Patients were examined for infection before ischemia using a standardized questionnaire and received daily clinical examination after stroke. In case of fever, standardized radiological and microbiological examinations were performed. Fever within 48 h after stroke was observed in 30 (25.2%) patients. The probable cause of fever was infective or chemical aspiration pneumonia (n=12), other respiratory tract infection (n=7), urinary tract infection (n=4), viral infections (n=3) or insufficiently defined (n=5). (One patient had two potential causes of fever.) In thirteen of these patients, infection was most probably acquired before stroke. Fever newly developed more often during day 1 to 2 than day 3 to 7 after stroke (P=0.016). Fever was associated with a more severe deficit on admission independent from age, vascular diseases and risk factors (odds ratio 9.6; 95% confidence interval 3.1-29). Fever is a frequent complication early after stroke and in the majority of cases, it can be explained by infection or chemical aspiration pneumonia. In about half of the infected patients, infection was most probably acquired before stroke. Fever was associated with a more severe neurological deficit on admission.