What subject lines and messages in an e-mailed invitation maximize cooperation for a web survey? Answering this question is ever more important as survey researchers increasingly turn to web surveys to reduce costs. We test respondents' preferences for subject lines and messages in an e-mailed invitation to participate in a Consumer Assessment of Healthcare Providers and Systems survey. Findings suggest that respondents prefer informative and straightforward subject lines that reference the purpose and the sponsor of the survey. Brief and uninformative subject lines should be avoided. In the body of the e-mail, respondents prefer messages that expand on the study's purpose and usefulness and extend a guarantee of confidentiality. Messages that encourage participation for the sake of the study's representativeness or accuracy are unpersuasive to potential respondents. Few differences emerged in preferences among demographic subgroups.