The radiological appearance of primary influenza pneumonia can be difficult to distinguish on chest x-ray from pulmonary edema, given the presence of perihiliar congestion and hazy opacification, at least in the lower lobes (Figure 1a,b) . Pleural effusions may also be present. Computed tomography scans ( Figure 2) can add further diagnostic insight and may be useful to differentiate primary viral pneumonia from bronchiolitis and interstitial pneumonias, which occur frequently in children and young adults but have a benign outcome. Concomitant myopericarditis should be excluded by echocardiography. Concurrent pulmonary emboli, as suggested by early case reports from hospitalized patients with pandemic influenza A H1N1v 2009 in the US [13] , may further contribute to clinical deterioration in some patients. However, the occurrence of concomitant pulmonary emboli has not been reproduced in other geographic regions so far.