Many numerical models of climate predict an increase in interannual precipitation variability given the ongoing changes in global atmospheric composition. Few empirical studies exist in which trends in observed temporal precipitation variability have been systematically analyzed. In this investigation, we examine trends in interannual precipitation variability over the past century for the conterminous United States. A variety of statistical methods are applied to the time series, and we consistently find upward trends (most are statistically significant) in seasonal variability. Although strong spatial gradients or clusters do not appear in the trend values, trends toward increasing variability were substantially higher in the low-sun season than in the high-sun season. Our results from the United States provide empirical support for the many modeling studies simulating an increase in precipitation variability given continued global warming.