Though the overreporting of voter turnout in the National Election Study (NES) is widely known, this article shows that the problem has become increasingly severe. The gap between NES and official estimates of presidential election turnout has more than doubled in a nearly linear fashion, from 11 points in 1952 to 24 points in 1996. This occurred because official voter turnout fell steadily from 1960 onward, while NES turnout did not. In contrast, the bias in House election turnout is always smaller and has increased only marginally. Using simple bivariate statistics, I find that worsening presidential turnout estimates are the result mostly of declining response rates rather than instrumentation, question wording changes, or other factors. As more peripheral voters have eluded interviewers in recent years, the sample became more saturated with self-reported voters, thus inflating reported turnout.