The clinical importance of human epidermal growth factor receptor-2 (HER2) in breast cancer is now clearly established, given that expression of this tumor marker is used to guide therapy and as a prognostic indicator. Despite its now routine evaluation in breast cancer patients, population-based data are lacking because information on HER2 status is not routinely collected in the majority of population-based cancer registries. We assessed the feasibility of collecting HER2 data and its completeness in three registries in the Surveillance Epidemiology and End Results (SEER) Program. Among a sample of invasive first primary breast cancer patients diagnosed between June and December 2007, HER2 tests had been done on 96.5% (n = 522), and test results were available for 95.2% (n = 515) of patients. The majority of HER2 tests were performed by immunohistochemistry alone (50.9%), 35.3% by both immunohistochemistry and fluorescence in situ hybridization (FISH), and 11.8% of tests by FISH alone. As a result of these findings, SEER registries will collect HER2 data on all invasive breast cancer patients as an optional data element for those diagnosed in 2009 and HER2 will likely be a required data element for these patients in 2010.