Montgomery County, Maryland was the first major jurisdiction to pass a law requiring smoke detectors in all homes. Smoke detector coverage in the county was evaluated five years after the law's implementation and compared to the coverage in neighboring Fairfax County, Virginia, which has no such law. Firefighters visited 651 randomly selected owner-occupied homes and tested each detector. While a similar percentage of homes in Montgomery and Fairfax counties complied with detector codes (42 per cent vs 44 per cent, respectively), Montgomery County had a significantly lower percentage of homes with no working detectors (17 per cent vs 30 per cent) and with no detectors at all (6 per cent vs 16 per cent). In general, Montgomery County residents complied with what they believed the law required, but lacked knowledge of the law's details. New homes where building codes required detectors and homes where owners assumed that detectors were required by law were likely to have working detectors. Analyses of 12 years of fire data suggest that as a county approaches complete detector coverage, the risk of residential fire deaths decreases. An essentially unenforced law seems to be obeyed because it conforms to community values.