Patients can obtain screening mammograms without a physician's referral, leading to potential problems in clinical care. Because of the complexity of self-referral and the limitations of prior studies, we examined this phenomenon in a representative sample of mammography facilities. A questionnaire was given to all women obtaining mammograms at nine Connecticut mammography facilities during a 2-week period. Facilities included mobile sites, urban fixed sites, and rural fixed sites. Responses were categorized according to whether or not the woman had seen a primary care provider within the last year and whether or not she had received a recommendation to obtain a mammogram. The response rate was 62% (732 of 1,189), and the mean age of respondents was 58 years (range, 30-100 years). Self-referred women, defined as those without a recent visit to a primary care clinician and without a clinician's recommendation for a mammogram, constituted 6% of respondents. Self-referred women were significantly more likely to use mobile facilities (78% vs 33%,p<.01) and be under 50 years of age (44% vs 28%,p=.02) compared with provider-referred women who had recently seen their provider. We conclude that younger women are obtaining screening mammograms without clear evidence of having seen their primary care provider in the previous year or having received a referral from their provider. Self-referral is especially common at mobile mammography facilities. Further study is needed to assess the clinical impact of self-referral on mass screening programs.