With the threat of avian influenza, influenza laboratory testing and surveillance capacity has increased globally. Data from global surveillance activities have been used to identify circulating influenza strains for vaccine policy decisions, and have provided evidence of influenza disease among various populations. A recent meta-analysis, which includes findings from these surveillance efforts, has shown that influenza contributes to 10% of pediatric respiratory hospitalizations. Although statistical models indicate a high burden of influenza-associated morbidity among older adults and pandemic studies reveal an increase in hospitalizations among young adults, the global burden of seasonal influenza among adults remains unknown. In order to estimate the global burden of seasonal influenza among adult respiratory hospitalizations, we conducted a systematic review of the published literature, and identified 48 eligible articles published between January 1996 and June 2012 that met our inclusion criteria. We combined these published datasets with 29 eligible, unique datasets from year-round, influenza hospital-based surveillance. These combined data covered 50 countries with varying income and vaccine policies. Extracting numbers tested and positive for influenza, we calculated crude median positive proportions and evaluated potential differences in crude proportions among variables using Kruskal-Wallis non-parametric tests. We observed differences by data source and country development status when we included the 2009 pandemic year. With the exclusion of the 2009 pandemic year, we then generated adjusted pooled estimates using the log binomial model. We found 11% of cases from adult respiratory hospitalizations worldwide were laboratory-confirmed for influenza. This pooled estimate was independent of age but increased as country development or income level decreased. Our findings suggest that influenza is an important contributor to severe acute respiratory illness among both young and older adult populations. For countries without reliable influenza data, we provide an estimate that they may use in planning and allocating resources for the control and prevention of influenza.