Surveillance of 2009 pandemic influenza A(H1N1) in Denmark was enhanced during the 2009–10 winter season with a system monitoring the burden of the pandemic on intensive care units (ICUs), in order to inform policymakers and detect shortages in ICUs in a timely manner. Between week 46 of 2009 and week 11 of 2010, all 36 relevant Danish ICUs reported in two ways: aggregate data were reported online and case-based data on paper. Cases to be reported were defined as patients admitted to an ICU with laboratory-confirmed 2009 pandemic influenza A(H1N1) infection or clinically suspected illness after close contact with a laboratory-confirmed case. Aggregate numbers of cases were reported weekly: during weeks 48-51 (the peak), reporting was daily. The case-based reports contained demographic and clinical information. The aggregate surveillance registered 93 new cases, the case-based surveillance 61, of whom 53 were laboratory confirmed. The proportion of beds used for influenza patients did not exceed 4.5% of the national capacity. Hospitals with cases used a median of 11% of bed capacity (range: 3–40%). Of the patients for whom information was available, 15 of 48 patients developed renal insufficiency, 19 of 50 developed septic shock and 17 of 53 died. The number of patients with pandemic influenza could be managed within the national bed capacity, although the impact on some ICUs was substantial. The combination of both reporting methods (collecting aggregate and case-based data) proved to be useful for monitoring the burden of the pandemic on ICUs.