This study aimed to assess covariates of drug use trajectories among 102 adolescents admitted to a drug user treatment program between November 2005 and November 2006 in Québec, Canada. The influences of mental health, therapeutic alliance, and treatment persistence were examined. The Addiction Severity Index was used to measure drug use severity and mental health problems; the California Psychotherapy Alliance Scales was used for therapeutic alliance. latent growth curve analysis showed associations between (1) mental health and initial drug use severity; (2) therapeutic alliance and initial drug use severity; and (3) number of post-treatment sessions attended and drug use severity over time.