In 1995 a group of 1724 randomly selected Minnesota taxpayers was informed by letter that the returns they were about to file would be ‘closely examined’. Compared to a control group that did not receive this letter, low and middle-income taxpayers in the treatment group on average increased tax payments compared to the previous year, which we interpret as indicating the presence of noncompliance. The effect was much stronger for those with more opportunity to evade; in fact, the difference in differences is not statistically significant for those who do not have self-employment or farm income, and do not pay estimated tax. Surprisingly, however, the reported tax liability of the high income treatment group fell sharply relative to the control group.