Purpose: The Medicare Consumer Assessments of Healthcare Providers and Systems (MCAHPS) survey, a primarily English-language mail survey with English and Spanish telephone follow-up, is the primary means of assessing the health care experiences of American seniors. We examine unit (whole survey) and item nonresponse for this survey to explore issues regarding surveying seniors about their health care. Design and Methods: We describe overall rates and analyze predictors of unit and item nonresponse for the 695,197 Medicare beneficiaries selected for the 2007 MCAHPS survey (335,249 unit respondents, 49% overall response rate). Results: Asians, African Americans, and Hispanics responded at adjusted response rates 7-17 percentage points lower than non-Hispanic Whites (p less than 0.001 for each). Among seniors, response rates dropped beyond age 75. Asians and older beneficiaries were especially likely to respond by mail, and African Americans and Hispanics by phone. Breakoff from telephone surveys was most common among African Americans and older respondents. Among respondents, older age was the strongest predictor of item missingness (e.g., those 85 years and older failed to answer items at twice the rate of those aged 65-74 years, p less than 0.001). Non-Hispanic Whites had lower rates of item missingness than other racial/ethnic groups (p less than 0.001 for each; one-third lower than African Americans). Implications: Survey research on older adults, especially regarding racial/ethnic disparities in health care, could benefit from improved response rates. These results suggest that targeted prenotification materials and campaigns, tailored follow-up, targeted Spanish mailings, Chinese translations/calls, and adjustments to telephone protocols may improve representation and response.