While the so-called “fillers” um and uh share a great deal in the way of interpretation, association, and usage, they are far from perfect substitutes. Previous corpus research, focusing primarily on British English, has identified a number of social and discursive factors with which filler usage can vary, including pause length and position in an utterance and speaker age, gender, and social class (Rayson et al. 1997, Clark and Fox Tree 2002, Tottie 2011, inter alia). Building on such research, the present paper investigates social variation in the use of um and uh in the United States. In particular, the paper documents the results of two corpus-based investigations of women’s and men’s usage of um and uh demonstrating that, among the speakers represented in the corpora, women on the aggregate had a far higher ratio of um tokens to uh tokens (um/uh ratio) than did men. The first of the two corpora examined is a collection of 992 transcripts from three speed-dating events held for graduate students at an American university in 2005. In this corpus, women’s average um/uh ratio is more than 3.5 times that of men. An analysis of gendered filler usage in the Switchboard Corpus (SWBC) yields a similar result: women’s average um/uh ratio in the SWBC is more than 2.5 times that of men. Data from the SWBC likewise suggest that this general trend persists across age groups and major U.S. dialect regions and, furthermore, tends to hold for speakers regardless of the gender of their interlocutors. The SWBC also provides evidence suggesting that um is gaining currency relative to uh; i.e., that there is a linguistic change in progress whereby the use of um relative to uh is on the rise. It is noted that not all men and women in the corpora exhibit filler usage in line with the aggregate-level trends, and that gendered linguistic differentiation should not be assumed to be a direct reflection of gender per se (Eckert 1989). A thorough understanding of the dynamics of gender and filler usage calls for an examination of the meanings and associations of um and uh and of speakers’ stances, objectives, and relation to their social world.