Beginning in December 2016, calorie labeling on menus will be mandatory for US chain restaurants and many other establishments that serve food, such as ice cream shops and movie theaters. But before the federal mandate kicks in, several large chain restaurants have begun to voluntarily display information about the calories in the items on their menus. This increased transparency may be associated with lower overall calorie content of offered items. This study used data for the period 2012-14 from the MenuStat project, a data set of menu items at sixty-six of the largest US restaurant chains. We compared differences in calorie counts of food items between restaurants that voluntarily implemented national menu labeling and those that did not. We found that the mean per item calorie content in all years was lower for restaurants that voluntarily posted information about calories (the differences were 139 calories in 2012, 136 in 2013, and 139 in 2014). New menu items introduced in 2013 and 2014 showed a similar pattern. Calorie labeling may have important effects on the food served in restaurants by compelling the introduction of lower-calorie items.