There has been much enthusiasm shown in the literature about Native Speaker-Nonnative Speaker (NS-NNS) e-mail interactions, associating them with increased motivation and participation and reduced anxiety (Beauvois & Eledge, 1996; Leh, 1997; Aitsiselmi, 1999). Recent research has now also begun to link these interactions with increases in L2 proficiency (Floréz-Estrada, 1995; Stockwell & Harrington, 2003). As some studies have suggested that L2 learners should reach a certain number of e-mail interactions in order for benefits to accrue (Lamy & Goodfellow, 1999; Stockwell & Harrington, 2003), researchers have started to turn their attention towards what factors play a role in helping to facilitate longer interactions. One factor that appears to be important in sustaining longer conversation sequences is the topics discussed during the interactions (Stockwell & Levy, 2001). While Lamy & Goodfellow (1999) investigated the topic threads during on-line discussions with multiple participants, threads in NS-NNS paired interactions through e-mail remain largely unexplored. Thus, in this study, 48 learners of Japanese involved in e-mail interactions with native speakers were investigated to determine what features of topic threads contributed to sustaining interactions. The end-of-thread messages (i.e., those messages which were the last message in a conversation thread) were examined in terms of whether or not a reply was invited, and those messages for which a reply was invited were further analysed to determine why a reply was not received. The paper concludes with suggestions for how conversation threads may be sustained, and some directions for future research.