In 2008, the New York State Department of Taxation and Finance sent letters to clients of a fraudulent tax preparer, warning them of a possible audit and asking them to participate in the Department’s Voluntary Disclosure and Compliance Program if they had filed inaccurate tax returns in the past. This study examines the impact of the letters on voluntary compliance in their future (2008 and 2009) returns. In this study, a simple method similar to “difference in differences”, which we call “difference in positions”, is applied. 10,000 samples are randomly drawn from the taxpayer population and the growth rates of Federal adjusted gross income (AGI) for these samples are put into relative frequency density graphs. We then examine the relative positions of the experiment group (the clients of the fraudulent tax preparer) within the normally distributed curves before and after the letters were sent. The change in the relative positions is regarded as the letter impact on voluntary compliance. It is found that the impact is significant in the first year (2008 tax returns) after the letters were sent. The impact is 17.49 percentage points on the AGI growth rate, which translates to $8.68 million of reported AGI for the 507 taxpayers in the experiment group. However, the impact is minimal in the second year (2009 tax returns), indicating that the long-run effect of the letter mailings may be weak.