A health maintenance organization (HMO)-based program designed to increase breast cancer screening was evaluated, focusing on changes in mammography utilization. The program consisted of a multistage intervention aimed at women members and primary care physicians of the HMO. This report examines the effect of the intervention on mammography utilization. The program was evaluated using a quasiexperimental design in which a random sample of women aged 50-74 from the HMO (intervention) was compared to a similarly aged geographic control group selected through random digit dialing. From 1988 to 1990, 450 intervention women and 450 control women were sampled (without replacement) each year and surveyed about breast cancer screening practices and related knowledge. A clear increase in self-reported mammography utilization was associated with the intervention. The percentage of women who reported a mammogram in the 12 months prior to the survey increased from 41% in 1988 (baseline) to 68% in 1990 among HMO women, compared to a change from 39% to 49% among control women. Comparing postintervention rates of mammography in HMO versus control women yielded a rate ratio (RR) of 1.4. However, this effect was strongly modified by income and race. Women with annual incomes of $31,000 or more showed little (whites, RR = 1.2) or no (blacks, RR = 1.0) effect of the intervention. Among women with incomes less than $31,000, the effect among whites (RR = 1.9) was much stronger than among blacks (RR = 1.2).