BACKGROUND: Credible measures of disease incidence, trends and mortality can be obtained through surveillance using manual chart review, but this is both time-consuming and expensive. ICD-10 discharge diagnoses are used as surrogate markers of infection, but knowledge on the validity of infections in general is sparse. The aim of the study was to determine how well ICD-10 discharge diagnoses identify patients with community-acquired infections in a medical emergency department (ED), overall and related to sites of infection and patient characteristics. METHODS: We manually reviewed 5977 patients admitted to a medical ED in a one-year period (September 2010-August 2011), to establish if they were hospitalised with community-acquired infection. Using the manual review as gold standard, we calculated the sensitivity, specificity, predictive values, and likelihood ratios of discharge diagnoses indicating infection. RESULTS: Two thousand five hundred eleven patients were identified with community-acquired infection according to chart review (42.0%, 95% confidence interval [95%CI]: 40.8-43.3%) compared to 2550 patients identified by ICD-10 diagnoses (42.8%, 95%CI: 41.6-44.1%). Sensitivity of the ICD-10 diagnoses was 79.9% (95%CI: 78.1-81.3%), specificity 83.9% (95%CI: 82.6-85.1%), positive likelihood ratio 4.95 (95%CI: 4.58-5.36) and negative likelihood ratio 0.24 (95%CI: 0.22-0.26). The two most common sites of infection, the lower respiratory tract and urinary tract, had positive likelihood ratios of 8.3 (95%CI: 7.5-9.2) and 11.3 (95%CI: 10.2-12.9) respectively. We identified significant variation in diagnostic validity related to age, comorbidity and disease severity. CONCLUSION: ICD-10 discharge diagnoses identify specific sites of infection with a high degree of validity, but only a moderate degree when identifying infections in general.