The World Trade Center Health Registry (WTCHR) is currently conducting its third wave (W3) of data collection. Wave 1 (W1), conducted between September, 2003 and November, 2004, yielded a 71,437 person cohort for understanding health effects from the September 11, 2001 terrorist attack on the World Trade Center. Wave 2 (W2) completed between November, 2006 and December, 2007 had 46,322 adult participants. W3, like W2, used mixed mode: internet, paper, and phone. A greater rate of completion for web surveys results in cost savings for paper and phone modes. This study assessed the impact of multiple email reminders on internet based Wave 3 survey response. The literature on email reminder effect on internet based survey is generally limited to only a few reminders within a set period of time (e.g. Cook, Heath & Thompson, 2000; Archer, 2007). 39,386 WTCHR enrollees who had verified email addresses were invited in batches to complete the W3 internet based questionnaire. Several weeks after the last batch of email invitations, the first reminder email was sent to non-responders to the invitation (n=33,456). Up to 12 reminders will be sent at 7-10 day intervals, on different days of the week, and with different subject lines. Daily response rates before and after email reminders were compared by using a difference ratio. The first email reminder had the greatest impact with a difference ratio of 21. After nine reminders the difference ratio was 4 indicating that even after multiple instances of contact, reminders continued to have a substantial impact on response rate. Further analysis will address other questions such as: a) What is the relative lag of impact on base response rate following each reminder; and b) How does email reminder impact vary by mode of participation in prior survey wave.