Eighteen months after the 1980 initiation of the Canadian National Breast Screening Study (NBSS) (a multicenter randomized controlled trial that will involve 90,000 women), the Toronto center experience such severe problems with recruitment that the study as a whole was jeopardized. Impediments to recruitment arose in five areas. The design of the protocol was criticized by some physicians and participants, a problem that could only be met by justifying the design. A second problem was a lack of understanding of the concept of screening and mistaken assumptions about what participation in the NBSS entailed. Thirdly, recruitment problems occurred at a time when the media were focusing much attention on the hazards of low-level ionizing radiation. Fourthly, women were found to have a variety of personal reasons for delaying or avoiding entry into the study. Finally, factors characteristic of the Canadian milieu such as universal health coverage may have acted as a disincentive to entry. To improve physician understanding, NBSS personnel made presentations at medical rounds and scientific meetings; articles were written for medical journals. To win support from the lay public, talks were given to recreation or work-based groups. Appearances on radio and television talk shows were sought out. Mass mailings to university staff and professional associations did not produce large responses, nor did advertisements on television, radio, or in newspapers. The distribution of a check insert in a government mailing gave rise to hundreds of appointments across Canada. However, for generating an ongoing adequate level of recruitment, the best measure has been the mailing of personally addressed letters to eligible women followed by a telephone call. Data on response rates, cost and women's attitudes toward the study are reported. By early 1983, Toronto met its recruitment target of 12,000.