OBJECTIVE: We examined how predisposing, enabling and reinforcing factors influence mammography referrals by primary care physicians (PCPs). METHODS: Using the 2001-2003 National Ambulatory Medical Care and National Hospital Ambulatory Medical Care Surveys, we identified visits to office (n=8,756) and outpatient (n=17,067) PCPs by women≥40 without breast symptoms or breast cancer. We examined mammography referrals by predisposing (age, race, ethnicity, education, chronic problem), enabling (income, payer, visits within 12 months, time with physician), and reinforcing factors (physician age, gender, specialty/clinic, PCP status, region, MSA, solo/group practice). Gender, specialty, physician age, time with physician and solo/group were only in NAMCS. Clinic type was only in NHAMCS. We fitted logistic regression models adjusted for all factors and year. RESULTS: Office-based referrals were more likely during visits: for preventive or chronic care; with private payer vs self/uninsured; by women with no visit within 12 months vs≥3; lasting≥15 minutes; to female PCPs; to PCPs aged ≥45; to gynecologists. Outpatient referrals were more likely during visits: by Hispanics; for preventive or chronic care; by women with no visit within 12 months; to one's own PCP; to gynecologic clinics; in the Northeast or Midwest. CONCLUSIONS: Reinforcing factors, in addition to predisposing and enabling factors, are associated with mammography referral. Interventions to increase referrals should consider provider factors and aspects of the healthcare environment, and recognize differences between settings. Efforts to facilitate referrals during chronic care visits or outpatient visits to non-PCP providers may provide opportunities to increase screening. Efforts are needed to ensure that uninsured women are receiving appropriate referrals.