BACKGROUND: Surveillance, Epidemiology, and End Results (SEER) registry data have been used to suggest underuse and disparities in receipt of radiotherapy. Prior studies have cautioned that SEER may underascertain radiotherapy but lacked adequate representation to assess whether underascertainment varies by geography or patient sociodemographic characteristics. The authors sought to determine rates and correlates of underascertainment of radiotherapy in recent SEER data. METHODS: The authors evaluated data from 2290 survey respondents with nonmetastatic breast cancer, aged 20 to 79 years, diagnosed from June of 2005 to February 2007 in Detroit and Los Angeles and reported to SEER registries (73% response rate). Survey responses regarding treatment and sociodemographic factors were merged with SEER data. The authors compared radiotherapy receipt as reported by patients versus SEER records. The authors then assessed correlates of radiotherapy underascertainment in SEER. RESULTS: Of 1292 patients who reported receiving radiotherapy, 273 were coded as not receiving radiotherapy in SEER (underascertained). Underascertainment was more common in Los Angeles than in Detroit (32.0% vs 11.25%, P