OBJECTIVES: While it is known that accurate and timely diagnosis and treatment of infections can present a major healthcare challenge, there is a lack of in-depth studies examining infection diagnosis and treatment practices and their associated costs in large real-world patient populations. The study objective was to examine infection prevalence, current diagnosis and treatment practices, and costs associated with managing patients with infections via retrospective analysis of a large US electronic health record database. METHODS: Patients with infections were identified based on the use of antimicrobial therapy for >24 h following hospital admission. Infection type was identified using ICD-9 diagnosis coding. Patients were classified into four groups: (1) pneumonia, (2) sepsis without pneumonia, (3) other infectious etiology, and (4) non-infectious etiology initially suspected as infectious. Illness severity was assessed using the Acute Physiology Score. Diagnostic procedures and treatments were identified via ICD-9 procedure coding and pharmacy order data, respectively. Per-patient cost estimates ($USD), calculated over 15 days from admission, encompassed diagnostic tests, radiologic procedures, and antimicrobial and other pharmaceutical treatments. RESULTS: In total, 127,174 patient visits met inclusion criteria. Pneumonia was the most common infection type (20.1% of visits), followed by other infectious etiology (16.0%) and sepsis without pneumonia (7.0%). Non-infections initially suspected as infections represented 56.9% of visits. Overall, 62.5% of patients had medium or high illness severity. Length-of-stay was >5 days for 49.2% of patients. Bacterial tests accounted for 92.4% of diagnostic tests, and antibiotics accounted for the majority (>98%) of antimicrobial treatments. Suspected infections with non-infectious etiology had the highest per-patient costs for low-severity ($14,228) and medium-severity patients ($12,551). For high-severity patients, pneumonia carried the highest per-patient cost ($14,541). CONCLUSIONS: This study demonstrates that the diagnosis and treatment of hospitalized patients with infections carries a significant cost and suggests potential benefits in reducing time to diagnosis.