The goal of this study was to develop an understanding the developmental trajectory of smoking behaviors in adolescents who seek smoking cessation treatment to inform tailored prevention and treatment efforts; this includes identifying gender differences in smoking behaviors. Smoking trajectory was examined retrospectively in 639 treatment-seeking adolescents (59% female; 44% African American, 50% European American, mean +/- SD daily cigarettes per day [CPD] 19.16 +/- 7.2 for both girls and boys). Smoking trajectory variables examined included age at first cigarette, age at daily smoking (a proxy measure for onset of dependence), and age at treatment request. The time interval from first cigarette to daily smoking was shorter for girls than for boys (mean +/- SD 0.9 +/- 1.1 years for girls, 1.3 +/- 1.5 years for boys, p < 0.01). From this clinical sample of adolescent smokers, findings suggest only a brief window of opportunity for secondary preventive interventions before the development of tobacco dependence. Additional research is needed to explore the specific factors that differentially affect smoking trajectory in girls compared to boys.