Hippocrates may not have foreseen the complexities of modern medicine, but 24 centuries ago he (or whoever wrote the Hippocratic Oath) set down a fundamental rule for all clinicians and researchers—“first do no harm.” This rule has special resonance this week as we explore over-diagnosis in breast cancer screening, and the evolving clinical trial nightmare at Northwick Park.

Three weeks ago an editorial in the BMJ concluded that despite limitations, breast cancer screening does save lives (BMJ 2006;332: 499-500). But in the same issue of the journal we published an analysis by Karsten Juhl Jørgensen and Peter …