The extent to which major cohort studies now rely on centralized machine-readable files of personal records is taken largely for granted by investigators, but not by the public, privacy advocates, politicians, or the popular press. When widespread linkage into personal histories for statistical studies was first envisaged a few decades ago, it was regarded as impractical. But privacy advocates now fear the statistical uses even more than the administrative applications, perhaps because the latter are inevitable to prevent abuses, and often are associated with monetary benefits or personal convenience. Indeed a concept of 'privacy' has arisen that is deemed to be violated even in statistical studies where confidentiality is assured, unless the particular purpose is approved by all individuals. This concept motivates much of the political thinking. What could be lost to preventive medicine and clinical testing by this is illustrated with data from cohort studies reported in this journal (CCC) in 1993.