Would holding elections by mail increase voter turnout? Many electoral reform advocates predict that mail ballot elections will boost participation, basing their prediction on the high turnout rate among absentee voters and on the rise in voter turnout after Oregon switched to voting by mail. However, selection problems inherent to studies of absentee voters and Oregon give us important reasons to doubt whether their results would extend to more general applications of voting by mail. In this paper, we isolate the effects of voting in mail ballot elections by taking advantage of a natural experiment in which voters are assigned in a nearly random process to cast their ballots by mail. We use matching methods to ensure that, in our analysis, the demographic characteristics of these voters mirror those of polling-place voters who take part in the same elections. Drawing on data from a large sample of California counties in two general elections, we find that voting by mail does not deliver on the promise of greater participation in general elections. In fact, voters who are assigned to vote by mail turn out at lower rates than those who are sent to a polling place. Analysis of a sample of local special elections, by contrast, indicates that voting by mail can increase turnout in these otherwise low-participation contests. © The Author 2007. Published by Oxford University Press on behalf of the Society for Political Methodology. All rights reserved.