OBJECTIVE: To describe the case characteristics and outcomes of patients hospitalised with pandemic (H1N1) 2009 influenza infection during the first 2 months of the epidemic. DESIGN, PARTICIPANTS AND SETTING: Prospective case series of 112 patients admitted to seven hospitals in Melbourne with laboratory-confirmed pandemic (H1N1) 2009 influenza between 1 May and 17 July 2009. MAIN OUTCOME MEASURES: Details of case characteristics, risk factors for severe disease, treatment and clinical course. RESULTS: Of 112 hospitalised patients, most presented with cough (88 ) and/or fever (82 ), but several (4 ) had neither symptom. A quarter of female patients (15) were pregnant or in the post-partum period. Patients presenting with multifocal changes on chest x-ray had significantly longer hospital lengths of stay, and were more likely to require intensive care unit admission. Thirty patients required admission to an intensive care unit, and three died during their acute illness. The median length of intensive care admission was 10.5 days (interquartile range, 5-16 days). CONCLUSIONS: This study highlights risk factors for severe disease, particularly pregnancy. Clinical and public health planning for upcoming influenza seasons should take into account the spectrum and severity of clinical infection demonstrated in this report, and the need to concentrate resources effectively in high-risk patient groups.