BACKGROUND. Early recognition of influenza virus infection in hospitalized patients can prevent nosocomial transmission. OBJECTIVE. To determine the clinical presentation of influenza in hospitalized patients. DESIGN. Case series. Data were collected retrospectively from medical records and included demographic information, comorbidities, clinical symptoms and signs, microbiologic test results, and outcomes (including pneumonia and intensive care unit [ICU] admission). SETTING. A 1,400-bed teaching hospital. PATIENTS. A total of 207 inpatients who received a diagnosis of influenza virus infection during 3 seasons from 2000 to 2003. RESULTS. Over the course of 3 seasons, 207 patients received a diagnosis of influenza (186 were infected with influenza A virus, and 21 were infected with influenza B virus). The most commonly reported symptoms were cough (186 patients [90%]) and subjective fever (137 patients [66%]); 124 patients (60%) had a documented temperature of 37.8°C or greater before influenza was diagnosed. Sore throat was uncommon (44 patients [21%]). Centers for Disease Control and Prevention (CDC) criteria for influenza-like illness (ILI)-temperature 37.8°C or greater and either cough or sore throat-were met by 107 patients (51%). There were no differences in the proportion of patients who met ILI criteria with respect to age, sex, season, influenza virus type, or time to diagnosis in the hospital. Most patients (150 [72%]) received acetaminophen. Only 41 patients (20%) had positive results of clinical cultures; 178 patients (86%) received antibiotic therapy. Fifty-six patients (27%) had pneumonia: 36 (17%) required admission to the ICU, and 25 (12%) required ventilatory support. Patients with pulmonary disease were more likely to require ventilatory support (12 [26%] vs 13 [8%]; P = .003). CONCLUSIONS. Only half of hospitalized patients with influenza met CDC criteria for ILI. These criteria may be more appropriate in outpatient settings. A high index of suspicion is needed to recognize influenza in hospitalized patients.