Colorado became the first state to make laboratory-confirmed influenza-associated hospitalizations a case-based reportable condition in 2004. We summarized surveillance for influenza hospitalizations in Colorado during the first 4 recorded influenza seasons (2004-2008). We highlight the similarities and differences among influenza seasons; no 2 seasons were entirely the same. The 2005-06 influenza season had 2 distinct waves of activity (types A and B), the 2006-07 season was substantially later and milder, and 2007-08 had substantially greater influenza B activity. The case-based surveillance for influenza hospitalizations provides information regarding the time course of seasonal influenza activity, reported case numbers and population-based rates by age group and influenza virus type, and a measure of relative severity. Influenza hospitalization surveillance provides more information about seasonal influenza activity than any other surveillance measure (e.g., surveillance for influenza-like illness) currently in widespread use among states. More states should consider implementing case-based surveillance for influenza hospitalizations.