This study examines effects of a randomized controlled trial for an online, family-based prevention program for older teens, Smart Choices 4 Teens, on alcohol use and related outcomes. Families (N = 411; teen age M = 16.4, SD = 0.5) were randomly assigned to the intervention or control condition in 2014-2015. Both intent to treat (ITT) and dosage models were conducted. ITT models: At the 6-month follow-up, teens in the experimental condition reported fewer friends who had been drunk, and parents in the experimental group reported more communication about social host laws. At the 12-month follow-up, parents in the experimental condition reported consuming fewer drinks than parents in the control group. Dosage models: At the 6-month follow-up, dosage was inversely related to teen drinking in the past 6 months or 30 days, frequency of teen drinking during the past 6 months and 30 days, drinks consumed by teens over the past 6 months, teen drunkenness and binge-drinking during the past 30 days, teen reported communication about safe drinking and positively related to parent and teen reported communication about social host laws. At 12 months, dosage was inversely related to teen alcohol use, frequency of teen drinking over the past 30 days, drinks consumed by teens over the past 6 months and 30 days, and teen drunkenness over the past 6 months. Results suggest that Smart Choices 4 Teens is beneficial for families. Dissemination and implementation strategies that motivate completion of program content will improve outcomes related to older teens' alcohol use. (PsycINFO Database Record (c) 2019 APA, all rights reserved).