The early trials of screening mammography, reporting 30% relative reduction in mortality from breast cancer in women over 50 years of age, led to strong professional and public demand for screening programmes. There has been little publicity about the subsequent trials showing no significant benefit in any age group, or about the harm and costs associated with screening mammography. For women under 50, there is a reluctant consensus that screening is not beneficial, but there is increasing pressure for publicly funded programmes for older women. When analysed in terms of population benefit, the randomised controlled prospective trials showed that the numbers of women screened to achieve one less death per year ranged from 7086 (Health Insurance Plan of New York), to 63,264 (Malmo), to infinity (Canadian National Breast Screening Study). About 5% of screening mammograms are positive or suspicious, and of these 80-93% are false positives that cause much unnecessary anxiety and further procedures including surgery. False reassurance by negative mammography occurs in 10-15% of women with breast cancer that will manifest clinically within a year. Our calculations confirm others that the mean annual cost per life "saved" is around $1-2 million (558,000 pounds). In the allocation of limited resources, public health policy on a proposed mass population intervention must be based on a critical analysis of benefits, harm, and cost. Since the benefit achieved is marginal, the harm caused is substantial, and the costs incurred are enormous, we suggest that public funding for breast cancer screening in any age group is not justifiable.