This study examines the use of uh and um — referred to jointly as UHM — in 14 conversations totaling c. 62,350 words from the Santa Barbara Corpus of Spoken American English. UHM was much less frequent than in British English with 7.5 vs. 14.5 instances per million words in the British National Corpus. However, as in British English the frequency of UHM was closely correlated to extra-linguistic context. Conversations in non-private environments (such as offices and classrooms) had higher frequencies than those taking place in private spaces, mostly homes. Time required for planning, especially when difficult subjects were discussed, appeared to be an important explanatory factor. It is clear that UHM cannot be dismissed as mere hesitation or disfluency; it functions as a pragmatic marker on a par with well, you know, and I mean, sharing some of the functions of these in discourse. Although the role of sociolinguistic factors was less clear, the tendencies for older speakers and educated speakers to use UHM more frequently than younger and less educated ones paralleled British usage, but contrary to British usage, there were no gender differences.