Objectives: In this study, we prospectively assessed changes in smoking a year after US adults' first-time purchase of a JUUL Starter Kit (JSK). Methods: Descriptive analyses assessed transitions in smoking status at 12 months among adult (age ≥21) JSK purchasers providing baseline and 12-month follow-up data (N = 27,164 [49.0% of baseline]), stratified by baseline smoking status. Baseline regular use of other ENDS was also considered. Results: Purchasers included baseline past 30-day smokers (65.2%), former smokers (12.0%), and never smokers (9.7%); over 90% were ever-smokers. One year later, the majority (58%) of those smoking at baseline reported no longer smoking. Former smokers or never smokers who were smoking at 12 months represented less than 2% of respondents. Former and current smokers who were regularly using other ENDS at baseline were less likely to report smoking at 12 months. Conclusions: Past 30-day smoking prevalence in a large longitudinal study of first-time JSK purchasers fell by more than half over 12 months. Analyzing the sample composition at purchase and transitions within subgroups defined by smoking status allows for a detailed understanding to help inform assessments of the population health impact of ENDS.