Critical decisions made 20 years ago by those who planned the randomized trial at the Health Insurance Plan (HIP) of Greater New York to determine the efficacy of periodic screening for breast cancer are detailed. These decisions affected the age group to be screened, screening modalities, frequency of screening, sample size, primary measures for testing efficacy, and period of follow-up (long term). Results of follow-up, 16 years after entry, indicate that mortality due to breast cancer continues to be lower among study women than controls. Numerically, the differential has been stable; relatively, it has decreased. It is estimated that the study group would have experienced about a 30% reduction in breast cancer mortality if screening had been maintained. Relative case survival rates over a 14-year period after diagnosis show changes in contours of trend lines that result from screening. The study group's trend is slightly concave in contrast to the usual convex curve for the controls. The contour of the curve is more decidedly concave among subjects detected through mammography alone than for other subgroups detected through screening, although the relative survival rate remains highest in the mammography only group. Uncertainty persists about effects of screening in the HIP study on breast cancer mortality among women aged 40-49 years at entry.