Results from the randomized trial conducted by the Health Insurance Plan (HIP) to determine the efficacy of breast cancer screening with mammography and palpation are reported for longer periods than previously available. By the end of 10 years after entry, the study group's mortality due to breast cancer was about 30% below the control group's. Arithmetic gains due to screening were maintained through year 14; relative gains declined. With increases in the period of follow-up, cumulative survival rates among cases detected by mammography alone (palpation negative during screening), decreased more rapidly than rates among other subgroups, but survival rates for mammography cases remained relatively high. Study women aged 40-49 years at entry began to show lower breast cancer mortality than those in the control group as duration of follow-up increased. Reservations are advanced about the acceptance of this finding as evidence of the efficacy of screening under age 50 under the conditions of the HIP study. The reservations are based on the observation that the decrease of mortality among the study group aged 45-49 at entry is concentrated entirely among cases diagnosed after they reached 50 years of age.