This study analyzes changes over a three-year period among Ontario retailers selling cigarettes to minors. Under supervision, 13 and 14-year-old minors were sent into stores to attempt to buy cigarettes. These minor-purchase-events (MPEs) were carried out in a local health unit that had implemented a community-based intervention and in an adjoining comparison health unit. After the local program we observed a large reduction (from 46% to 6%) in merchants willing to sell tobacco to minors. In the neighbouring health unit, a high rate of selling continued until a federal program using a similar intervention was implemented, after which a large reduction (from 47% to 2%) was observed. This magnitude of change has been unprecedented, except when active enforcement was implemented by police officers. Thus, from a public health perspective, it is important to understand what is influencing the store operators.