The objective of this study was to assess compliance with Boston's 100% smoke-free bar regulation within the first 3 months of implementation. An observational survey was conducted of a random sample of 102 bars in Boston before and after the smoking ban went into effect. Forty bars were observed both pre- and postban, serving as a true comparison group; 62 additional bars were observed postban only. From preban to postban, highly significant decreases were found in the mean proportion of patrons smoking inside (22.5% to 0.19%, p < 0.0001), in the proportion of bars with smoking patrons (100% to 2.5%, p < 0.0001), and in the average number of ashtrays present in each bar (24 to 0, p < 0.0001). A highly significant increase was found in the average number of "no smoking" signs posted in each bar (0 to 3.3, p < 0.0001). Within the 3 months immediately following the 100% smoke-free bar regulation in Boston, bars were highly compliant with the ban, including not allowing patrons and employees to smoke, removing ashtrays, and posting signs indicating that smoking is prohibited. Therefore, with proper time and preparation, public health practitioners can change social norms before a ban goes into effect and can implement smoke-free policies smoothly and with immediate compliance.