Introduction: To examine whether changes in select measures of e-cigarette puffing topography are associated with changes in smoking behavior. Methods: Sixteen current cigarette smokers were instructed to completely switch from smoking combustible cigarettes to using e-cigarettes over a 2-week period. The study was completed in the Southern Midwestern region of the United States. Measures included demographics, smoking history, and cigarette dependence, as well as baseline and 2-week follow-up self-reported cigarettes per day, cigarette craving and urges, exhaled carbon monoxide readings, and e-cigarette usage data (puff number, puffing time, and average puff duration) collected via the e-cigarette built-in puff counter. Results: Over the 2-week switching period, participants significantly reduced their cigarettes per day (~80% reduction, p <. 0001). Although the number of e-cigarette puffs/day remained relatively stable (p >. 05), the average total e-cigarette daily puffing time increased significantly (p =. 001). Users' average puff duration increased by 91 ms/puff/d (p <. 001). The percentage decrease in cigarettes smoked per day was significantly and directly related to the slope of subjects' average puff duration over time (r(13) =. 62, p =. 01), such that as cigarettes per day decreased, puff duration increased. Self-reported smoking urges remained relatively stable from baseline to the end of the 2-week period (p >. 05). Conclusions: Among smokers switching to an e-cigarette, greater increases in e-cigarette puff duration was associated with greater reductions in cigarette smoking. Implications: The current study is one of the first to examine changes in smokers' e-cigarette puffing behavior and associated changes in cigarette consumption as they attempt to completely switch to e-cigarettes. During a 2-week switching period, participants reduced their cigarettes per day. Moreover, although e-cigarette puffs per day remained relatively stable, users' average puff duration increased significantly. Greater increases in e-cigarette puff duration were associated with greater reductions in cigarette smoking. Understanding how to effectively use an e-cigarette to best reduce and eventually quit smoking will be necessary as smokers increasingly turn to these products to facilitate possible cessation.