The carbon emissions of global healthcare activities make up 4%–5% of total world emissions, with the majority coming from industrialised countries. The solution to healthcare carbon reduction in these countries, ostensibly, would be preventive healthcare, which is less resource intensive than corrective healthcare in itself and, as a double benefit, reduces carbon by preventing diseases which may require higher healthcare carbon to treat. This leads to a paradox: preventive healthcare is designed to give humans longer, healthier lives. But, by extending (...) life spans, the carbon emissions of a person increase both over a lifetime and in the medical industry overall. At the same time, the need for higher carbon end-of-life care does not disappear, particularly in resource intensive countries. This article will first identify sources of healthcare carbon, focusing on the industrialised world, and explain various efforts towards healthcare carbon reduction, which include preventive healthcare. Second, it will develop the ‘paradox of prevention’—that preventive healthcare may save healthcare carbon by proximally reducing the need for medical treatments, but also, paradoxically, result in more healthcare carbon both in an individual’s life and in the medical industry. The third section will offer ethical principles for approaching the paradox of prevention. The conclusion will emphasise the need for institutional approaches to healthcare carbon reduction in the industrialised world, which will relieve some of the tensions of healthcare industry decarbonisation and individual healthcare carbon use. (shrink)