Background. Colorado is among the first states to legalize the recreational sale of marijuana and therefore among the first to develop regulations outlining the purchase, possession, consumption, and enforcement, and penalties. Colorado legislators set aside funds for a statewide informational media campaign to educate Colorado residents on legal use of marijuana. Method. This study evaluated the effectiveness of the mass media campaign to increase awareness of the law through a prospective cohort surveyed before and 6 months after the launch of the campaign (n = 798). Results. A total of 28.0% reported recall of the Good to Know campaign. There was a significant positive change in accurate knowledge of marijuana laws from baseline to follow-up for each of the four primary indicators of knowledge of the marijuana laws. Among those who had inaccurate knowledge of the laws at baseline, those who reported seeing the campaign at least once or more were 2.53 (95% confidence interval = 1.29-4.95) times as likely to report accurate knowledge of the laws at follow-up compared with those who did not recall seeing the campaign, particularly among marijuana users. Conclusions. Those individuals who reported recall of the campaign were more likely to increase their accurate knowledge of marijuana laws.