AIMS: To identify state, local and organizational characteristics associated with local law enforcement agencies' implementation of two dramatically different approaches to enforcement of underage drinking laws: compliance checks and Cops in Shops programs. Compliance checks use underage decoys to attempt to purchase alcohol from retail merchants, while Cops in Shops programs deploy undercover law enforcement officers in alcohol outlets to detect and cite persons under the age of 21 who attempt to purchase alcohol. DESIGN: Cross-sectional telephone interview conducted as part of the Tobacco Enforcement Study (TES), which examined enforcement of laws related to youth access to tobacco. SETTING: Data were collected in 1999 among law enforcement agencies in all 50 states of the United States. PARTICIPANTS: Representatives of city police departments, departments of public safety, sheriffs or county police were included (n = 920 local agencies). MEASUREMENTS: Alcohol compliance checks and Cops in Shops programs were the primary outcomes. Covariates included state level policies (e.g. beer tax), agency resources (e.g. number of sworn officers) and community demographics (e.g. college dormitory population). FINDINGS: Local enforcement agencies were more likely to perform alcohol compliance checks than to have a Cops in Shops program (73.9% compared to 41.1% in cities > 25 000 and 55.7% compared to 23.9% in cities < or = 25 000). Conducting compliance checks for tobacco age-of-sale laws was positively associated with alcohol compliance checks and Cops in Shops (OR 3.30, P < 0.001; OR 1.84, P = 0.001, respectively). Having a Drug Abuse Resistance Education (DARE) officer was negatively related to conducting compliance checks (OR 0.67, P = 0.03). Special community policing units were associated with departments having Cops in Shops programs (OR 1.80, P = 0.006). CONCLUSIONS: This study used a nationally representative sample of communities to better understand state and local factors that shape local law enforcement agencies' use of two distinct approaches to underage drinking enforcement. The strong link observed between tobacco and alcohol compliance checks may indicate a culture within some law enforcement agencies supporting strict enforcement of age-of-sale laws.