With increasing evidence of the effectiveness of mass screening in reducing mortality from breast cancer, the Breast Cancer Screening Study of the Health Insurance Plan of Greater New York is reviewed for its impact on increased efforts to gain participation among its target population. Initial contact efforts brought a 47% response rate, and further efforts increased this to 65%. The additional contact efforts increased response rates among most demographic groups. Women requiring minimum contact efforts were appreciably more likely than the increased effort group to participate in the full set of four screenings. Among those requiring increased efforts, more than 40% had the full set of screening examinations, and about 80% had at least one screening examination in addition to the baseline. Differences in breast cancer detection rates among contact effort groups were small, and there was little difference in mortality from breast cancer. Mortality rates from other causes, however, were higher among reluctant participants and highest among women who refused screening. Efforts to increase participation in screening for breast cancer should be linked to women's concerns with other health problems and to their medical care system.