["Low rates of influenza immunization among health care workers (HCWs) pose a potential health risk to patients in primary care practices. Despite previous educational efforts and programs to reduce financial barriers, HCW influenza immunization rates remain low. Variation in practice-level organizational culture may affect immunization rates. To explore this relationship, we examined organizational cultures and HCWs' influenza immunization behaviors in three family medicine practices.", 'We used a multi-method comparative case study. A field researcher used participant observation, in-depth interviews, and key informant interviews to collect data in each practice in November-December 2003. A diverse team used grounded theory to analyze text data.', 'Organizational culture varied among practices and differing HCW immunization rates were observed. The most structured and business-like practice achieved immunization of all HCWs, while the other two practices exhibited greater variation in HCW immunization rates. Physicians in the practices characterized as chaotic/disorganized or divided were immunized at higher rates than other members of the practices.'] Does organizational culture influence health care workers ' influenza immunization behavior?