OBJECTIVE To examine smoking trajectories in a clinical sample of adolescent smokers seeking cessation treatment, including: (a) smoking onset (initial, daily) and time intervals from initial to daily smoking and from daily smoking to treatment request, (b) associations between current level of tobacco dependence and smoking history, and (c) differences in smoking trajectory between African-American and non-African-American youth. METHODS Four hundred and thirty-two adolescent smokers (aged 13-17 years, 61.8% female, 32% African-American) responding to various media advertisement completed a telephone interview as part of pre-eligibility screening for a smoking cessation trial. Smoking trajectory data included age at onset of initial and daily smoking, intervals between those time points, and cigarettes smoked per day (CPD). Tobacco dependence was assessed using the Fagerström Test for Nicotine Dependence (FTND). Data were analyzed using regression models and multiple analyses of covariance. RESULTS Initial smoking occurred at a mean age of less than 12 years and daily smoking at age 13 years. Earlier onset of daily smoking was associated with higher FTND scores and longer duration from daily smoking to treatment request. For the entire sample, the time interval from initial to daily smoking was 1.14 years. When the sample was divided into early (before age 14 years) and later (at or after age 14 years) initiators, early initiators showed a slower progression from initial to daily smoking compared with late initiators (16 months vs. 6 months). Compared with non-African-American teen smokers, African-American youth reported a 1-year delay in onset of both initial and daily smoking. CONCLUSIONS Early age of daily smoking and short time interval from initial to daily smoking highlight a brief window of opportunity to prevent the development of tobacco addiction and its consequences. Ethnic differences in smoking trajectory uncovered in this report call for ethnically tailored interventions to reduce youth smoking.