BACKGROUND: Fever develops in 70% of ICU patients. In the present study, we tried to figure out causes of fever and the prognosis of febrile patients in the ICU in a prospective, cohort method. METHODS: From February to June 2007, patients admitted to medical ICU were daily screened and those who developed fever were enrolled. 237 consecutive admissions of 237 patients over a 5-month period were analyzed. Clinical parameters, including demographic data, underlying diseases, duration of ICU stay, causes of fever and final outcome were analyzed. RESULTS: Fever (core temperature > or =38.3degrees C) was present in 8% of admission, and it was caused by infective (84.2%) and non-infective processes (15.8%). Most fever occurred within first 5 days in the course of the admission (68.4%) and most lasted less than 5 days (57.9%). The median Acute Physiology and Chronic Health Evaluation (APACHE) III score at the time of fever was 43 (+/-19). Those with infectious fever had no significant differences in terms of severity of diseases in comparison with those with non-infectious cause of fever. The most common cause of infective fever was pneumonia (n=11). Prolonged fever (> or =5 days), all of which was caused by infection, occurred in 11 patients. Those with prolonged fever had higher mortality rate than short duration of fever (37.5% vs 0%, p<0.05). CONCLUSION: Infection, especially pneumonia is common cause of fever in the ICU. Prolonged fever is associated with high mortality rate.