BACKGROUNDPlanning for the treatment of infection with the 2009 pandemic influenza A (H1N1)virus through health care systems in developed countries during winter in the NorthernHemisphere is hampered by a lack of information from similar health caresystems.METHODSWe conducted an inception-cohort study in all Australian and New Zealand intensivecare units (ICUs) during the winter of 2009 in the Southern Hemisphere. We calculated,per million inhabitants, the numbers of ICU admissions, bed-days, and daysof mechanical ventilation due to infection with the 2009 H1N1 virus. We collecteddata on demographic and clinical characteristics of the patients and on treatmentsand outcomes.RESULTSFrom June 1 through August 31, 2009, a total of 722 patients with confirmed infectionwith the 2009 H1N1 virus (28.7 cases per million inhabitants; 95% confidenceinterval [CI], 26.5 to 30.8) were admitted to an ICU in Australia or New Zealand. Ofthe 722 patients, 669 (92.7%) were under 65 years of age and 66 (9.1%) were pregnantwomen; of the 601 adults for whom data were available, 172 (28.6%) had abody-mass index (the weight in kilograms divided by the square of the height inmeters) greater than 35. Patients infected with the 2009 H1N1 virus were in the ICUfor a total of 8815 bed-days (350 per million inhabitants). The median duration oftreatment in the ICU was 7.0 days (interquartile range, 2.7 to 13.4); 456 of 706 patients(64.6%) with available data underwent mechanical ventilation for a medianof 8 days (interquartile range, 4 to 16). The maximum daily occupancy of the ICUwas 7.4 beds (95% CI, 6.3 to 8.5) per million inhabitants. As of September 7, 2009,a total of 103 of the 722 patients (14.3%; 95% CI, 11.7 to 16.9) had died, and 114(15.8%) remained in the hospital.CONCLUSIONSThe 2009 H1N1 virus had a substantial effect on ICUs during the winter in Australiaand New Zealand. Our data can assist planning for the treatment of patients duringthe winter in the Northern Hemisphere.