BACKGROUND: Previous research has identified the impact of law enforcement practices on the behaviours and health of injection drug users (IDUs). We undertook a qualitative study of IDUs' experiences of policing practices in two Mexican cities on the U.S. border. METHODS: In 2004, two teams of Mexican interviewers conducted in-depth interviews with IDUs residing in Tijuana and Ciudad Juarez (Cd. Juarez), Mexico, who had injected drugs at least once in the prior month. Topics included types of drug used, injection settings, access to sterile needles and experiences with police. Field notes and transcribed interviews were analysed to identify emergent themes. RESULTS: Amongst the 43 participants, most reported that it is common for IDUs to be arrested and detained for 36h for carrying sterile or used syringes. Most reported that they or someone they knew had been beaten by police. Interviews suggested five key themes relating to police influence on the risk environment: (1) impact of policing practices on accessibility of sterile syringes, (2) influence of police on choice of places to inject drugs (e.g., shooting galleries), (3) police violence, (4) police corruption and (5) perceived changes in policing practices. CONCLUSION: Findings suggest that some behaviour of police officers in Tijuana and Cd. Juarez is inconsistent with legal norms and may be negatively influencing the risk of acquiring blood-borne infections amongst IDUs. Implementing a comprehensive and successful HIV prevention programme amongst IDUs requires interventions to influence the knowledge, attitudes and practices of law enforcement officers.