The evidence supporting breast cancer screening can be based on two controlled randomized trials in which population screening by mammography (and in one by clinical examination) was offered on a random basis (HIP: Shapiro et al. 1982; Four-County: Tabar et al. 1985), and two case-control studies in which the screening history of women dying from breast cancer was compared with that of healthy controls (Nijmegen: Verbeek et al. 1984; DOM: Collette et al. 1984). The question is whether breast cancer is always systemic or whether it is sometimes localized in cases of smaller tumors, in which case early detection should improve survival.