Routine mammographic screening increases detection of nonpalpable breast cancer. Timely follow-up of abnormalities is essential because delays may lead to postponement of treatment and decreased survival for women who have cancer. The purpose of this study was to determine the percentage of women with an abnormal mammogram who do not have adequate follow-up and to determine factors associated with inadequate follow-up. The study was conducted in a metropolitan health system that includes a large urban teaching hospital in Detroit and 26 ambulatory care centers. From the radiology database, all women with an abnormal screening mammogram performed between January 1, 1992, and July 31, 1992 were identified. We defined adequate follow-up as follow-up within three months of due date. Follow-up status was determined using medical records and telephone interviews. The percentage of women with inadequate follow-up was calculated. Relative risks compared percentages of women with inadequate follow-up according to demographic and screening-related variables. We calculated adjusted relative risks using multivariate binomial regression. We identified 1,249 women with abnormal screening mammograms. Inadequate follow-up occurred for 226 (18.1%) of the women. Among women with follow-up recommended in 4-6 months, 36.8% had inadequate follow-up. Among women with immediate follow-up recommended (obtain additional views or outside films for comparison, ultrasound, biopsy, or surgical referral), 7.2% had inadequate follow-up. Inadequate follow-up was associated with lower estimated household income and no history of previous mammogram. Among women with inadequate follow-up who were interviewed, 87% reported that they had been notified of their results. We found that the percentage of women with inadequate follow-up of abnormal mammograms is high, especially among women who require six-month follow-up. Women with low income and no history of a previous mammogram were at greatest risk for inadequate follow-up. These results document a previously unrecognized problem with mammography screening and suggest that the implementation of tracking systems to ensure timely follow-up of abnormal screening mammograms is essential. Medical Subject Headings (MeSH): mammography, follow-up, screening.