Purpose: Little is known about whether adolescent cigarette and e-cigarette use patterns over time differ by ethnicity. Methods: Data were pooled from three prospective cohort studies of adolescents in California and Connecticut (baseline: 2013e2014; 12-month follow-up: 2014e2015; N 6,258). Adjusted polytomous regression models evaluated the association of baseline exclusive ever e-cigarette use, exclusive ever cigarette use, ever use of both e-cigarettes and cigarettes (dual use) with past 30day use at follow-up (exclusively e-cigarettes, exclusively cigarettes, dual use; no use at baseline/ follow-up were the referent groups). Interaction analyses evaluated differences by race/ ethnicity (Hispanic white [HW], non-Hispanic white [NHW], Other). Results: A significant global interaction was observed for the association of baseline with followup tobacco use by ethnicity (p .009). Among NHW participants, ever e-cigarette or cigarette users at baseline (vs. never users) had significantly higher odds of every past 30-day use tobacco use pattern at follow-up. Among HW participants, compared with never users, exclusive e-cigarette users at baseline had increased odds of continued e-cigarette use (ORexclusive e-cigarettes 5.22; 95% confidence interval [CI]: 3.50, 7.79; ORdual use 3.64; 95% CI: 1.62, 8.18) but not of transition to exclusive cigarette use at follow-up (ORexclusive cigarettes 1.27; 95% CI:.47, 3.46), and HW exclusive cigarette users at baseline had greater odds of continued cigarette use (ORexclusive e-cigarettes 12.3; 95% CI: 5.87, 25.8; ORdual use 3.82; 95% CI: 1.06, 13.7) but not of transition to exclusive e-cigarette use at follow-up (ORexclusive cigarettes 1.61; 95% CI:.62, 4.18). Conclusions: Findings that NHW youth report more transitional use patterns and HW youth report more stable use patterns suggest a potential for differential impacts of e-cigarettes, by ethnicity, in increasing subsequent transition to or cessation from cigarette smoking. (C) 2019 Society for Adolescent Health and Medicine. All rights reserved.