The 2008 ACS additional mailing test tested the effectiveness of incorporating an additional reminder postcard or an additional replacement questionnaire to boost the mail response for nonrespondents without known telephone numbers and to assess the cost implications. Through the additional mailing test, we demonstrated that by adding either an additional replacement questionnaire or an additional reminder postcard we can increase the ACS mail return rate for the universe of nonrespondents without telephone numbers by about 6 to 7 percentage points. Furthermore, we demonstrated that the additional mailings boost the overall ACS mail response rate by about 1.6 to 1.8 percentage points. While the data from our study showed no evidence of a superior additional mailing method for boosting response between the additional replacement questionnaire and the additional reminder postcard, we did observe a difference in the operational and material costs of implementing the two additional mail strategies. The additional reminder postcard was the clear winner in terms of covering the cost of implementing the additional mail strategy as well as saving money. The net cost savings for the additional reminder postcard was estimated to be approximately $73,000 for the data collection month of March and cautiously extrapolating this result to a per annum basis, we estimate a potential yearly savings of approximately $875,000. This research demonstrates that this new method of targeting a subgroup of nonrespondents can work in the ACS environment. Given that both additional mailings produce a similar effect on mail response in the ACS, we have to take into account the cost implications of each method. As a result of the cost savings due to the postcard and the cost expenditure due to the questionnaire, we recommend implementing the additional reminder postcard into the production ACS mailing strategy. In addition to identifying the costs and benefits of mailing an additional reminder postcard, further evaluation is needed to quantify the benefits in reliability due to the increase in response. Given the positive results of the additional mailing test, we hope that through continued innovations in mail data collection methods, such as providing an additional mailing, we can continue to combat the recent trends in declining response to mail surveys as well as reduce the cost of expensive nonresponse followup operations used to supplement mail surveys.