Background:
Electronic cigarettes (e-cigarettes) are the most commonly used tobacco product among U.S youth. We used cross-sectional, nationally representative data of U.S. middle and high school students to examine the association between e-cigarette use and cigarette smoking initiation.

Methods:
Data were from the 2015 and 2016 National Youth Tobacco Surveys, a school-based survey of U.S. 6^th-12^th graders (pooled N=38,386). Questions on current age and age at initiation of different tobacco products were used to assess temporality. The study included 35,775 students who had never smoked conventional cigarettes five years before the survey (i.e., baseline), including never-smokers and those who first smoked < 5 years ago. Baseline never smokers were classified by e-cigarette use status into: (1) those who ever used e-cigarettes on/before or without ever smoking cigarettes; or (2) those who had never used e-cigarettes, or started only after initiating cigarette smoking. The outcome variables were cigarette smoking at pre-determined periods: any time within the past 5 years; past 1 year; past 6 months; past 30 days; and past 7 days. Adjusted odds ratios (AOR) were calculated using multivariable logistic regression. The models controlled for socio-demographic characteristics and use of smokeless tobacco, cigars, and hookah on/before cigarette smoking initiation.

Results:
Among baseline never cigarette smokers, 17.4% used e-cigarettes, and 16.7% initiated cigarette smoking within the past 5 years. Those who used e-cigarettes on/before ever smoking cigarettes had higher odds of smoking cigarettes than those who did not at all periods assessed: any time within the past 5 years (AOR=2.61); past 1 year (AOR=3.18), past 6 months (AOR=2.59), past 30 days (AOR=1.75), and past 7 days (AOR=1.38) (all p< 0.05).

Conclusions:
These cross-sectional findings reveal that e-cigarette use was associated with conventional cigarette smoking initiation among U.S. youth. Comprehensive efforts are important to prevent and reduce all forms of youth tobacco product use.