In this mass screening mammography program, self-referred women are accepted for screening only if part of a sponsoring organization. Screening is performed with mobile equipment, often at the work site. Of the first 22,540 screenings, 61% (n = 13,784) were of women aged 35-49 years. Eighty-nine percent (n = 20,025) of the screenings were normal. Of 51 women in whom cancer was found, 38 (75%) were recommended for biopsy on the basis of their initial two-view mammogram. The prevalence of breast cancer generally increased with advancing age but was similar between women aged 35-39 years and those aged 40-49 years. Of the 28 women with complete histologic studies, 43% (n = 12) had pure intraductal cancer and another 43% had negative findings at axillary node dissection. Program costs dictated a $65 fee. Within the design of this program it has been possible to screen large numbers of self-referred women and detect early carcinomas. Program design, however, may make screening more expensive than in a fixed screening site accepting only physician-referred women.