OBJECTIVE: To assess nonresponse bias in a mixed-mode general population health survey. DATA SOURCES: Secondary analysis of linked survey sample frame and administrative data, including demographic and health-related information. STUDY DESIGN: The survey was administered by mail with telephone follow-up to nonrespondents after two mailings. To determine whether an additional mail contact or mode switch reduced nonresponse bias, we compared all respondents (N = 3,437) to respondents from each mailing and telephone respondents to the sample frame (N = 6,716). PRINCIPAL FINDINGS: Switching modes did not minimize the under-representation of younger people, nonwhites, those with congestive heart failure, high users of office-based services, and low-utilizers of the emergency room but did reduce the over-representation of older adults. CONCLUSIONS: Multiple contact and mixed-mode surveys may increase response rates, but they do not necessarily reduce nonresponse bias.