Public health planners around the world had been awaiting the preliminary results of the randomized, controlled trial of breast cancer screening performed by the Canadian National Breast Screening Study (CNBSS) during the 1980s. It had been hoped that this large study would answer, with statistical validity, many of the questions concerning breast cancer screening that had not been answered satisfactorily by previous studies. Among the major unresolved issues is the desire to establish "absolute" proof that mammographic screening can benefit women who are between 40 and 49 years old.