Influenza RNA in blood (viremia) was detected in 9 of 79 (11.4%) hematopoietic cell transplant recipients with influenza, and was less frequently observed in patients with upper respiratory tract disease only and more frequently in patients infected with 2009 pandemic influenza A/H1N1 strain (versus seasonal strains). Viremia increased the risk of progression to lower respiratory tract disease (LRD), hypoxemia, respiratory failure, and overall and influenza-related death. Among patients with LRD, viremia was associated with increased hazards of overall and influenza-associated death (hazard ratio 3.5, 1.1-12). Thus, influenza viremia may serve as marker for overall poor outcome.