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Background: West Nile fever, considered a nonsevere manifestation of West Nile virus infection, has not been clinically well described in the United States. In 2002, Illinois had 884 documented cases of West Nile virus infection with 66 associated deaths., Objective: To describe the symptoms and functional outcomes of West Nile fever., Design: Case series., Setting: Illinois., Patients: 98 community-dwelling patients with laboratory evidence of West Nile virus infection but no history of clinical evidence of meningitis, encephalitis, or acute flaccid paralysis., Intervention: Outpatient interviews., Measurements: Presence and duration of patient-reported symptoms of infection, symptom-associated absenteeism, health care use, and impact on daily activities., Results: Of 98 patients, 96% had fatigue for a median of 36 days, 81% had fever for a median of 5 days, 71% had headache for a median of 10 days, 61% had muscle weakness for a median of 28 days, and 53% had difficulty concentrating for a median of 14 days. Thirty respondents reported hospitalization, with a median stay of 5 days. At 30 days after onset, 63% of respondents continued to have symptoms. Duration did not vary significantly with increased age. Among the 72 patients who normally attended work or school, 57 (79%) could not attend because of illness (median absence, 10 days)., Limitations: Recall bias could have been introduced by the delay between illness onset and interview and by self-reporting of illness information., Conclusions: West Nile fever is a more severe illness than has previously been documented. Mandatory reporting of West Nile fever cases in addition to West Nile meningoencephalitis cases could allow more accurate and timely recognition of the geographic distribution of West Nile virus infections and could inform public health interventions.