Cell phone-based ecological momentary assessment of substance use context for Latino youth in outpatient treatment: Who, what, when and where.

•Mobile assessment is promising for self-monitoring in adolescent drug treatment.•A better understanding of context is needed to fully utilize mobile self-monitoring.•Our study examined context through mobile assessment in Latino youth.•Being with close friends and hanging out are associated with adolescent drug use.•Findings inform development of mobile interventions triggered by social context.