Objectives: The Adult JUUL System User Switching and Smoking Trajectories (ADJUSST) study assessed the smoking and JUUL use trajectories of adults who purchased JUUL. In this paper, we describe study methods, characterize the sample, and assesses potential for bias due to loss to follow-up. Methods: We entered 55,414 US adults (≥ age 21) who purchased a JUUL Starter Kit for the first time (online or at retail) in 2018 into a naturalistic, longitudinal observational study, irrespective of baseline smoking status. Participants were invited for follow-ups 1, 2, 3, 6, 9, and 12 months later, focused on assessing past-30-day smoking and JUUL use. Analyses assessed potential bias due to non-response. Results: Over 90% of participants had a history of smoking; 62.8% were past-30-day smokers; 23.3% were former smokers. Participants' average age was 30; 75% were white. Most participants (77.6%) completed some follow-ups; 25% completed all follow-ups. Baseline differences among complete responders (N = 13,729), partial responders (N = 29,252), and complete non-responders (N = 12,433) were small. When recontacted, few 12-month non-responders said their non-response was due to smoking; many reported no past-30-day smoking. Conclusions: The study may elucidate smoking trajectories of adult JUUL users. The potential for bias due to loss to follow-up in ADJUSST was limited.