 The study estimates that healthcare encounters and costs attributable to influenza viral strains A and B in the US during the 2001-2002-2008-2009 seasons ranged from 11.3 to 25.6 million and 0.8 billion, respectively. Type B strains accounted for 37% of healthcare costs across all seasons, and as much as 66% in a single season. The outpatient burden of type B disease was considerable among persons aged 18 to 64 while the hospital cost burden was highest in young children. Increasing influenza vaccine coverage could potentially reduce overall annual influenza burden in the US. This article was authored by Song Kai-yan, Derek Waker, and Stefanie Sokolowski.