Whether the prices of certain orphan treatments are justified is highly controversial. One argument is that such therapies should not be funded through the public purse or private health plans because a patient with a rare disease requires more than their ‘fair share' of a limited health care budget. Orphan medications can also be denied because they fare poorly in the cost-effectiveness assessments of drugs. This paper takes the unusual line that life-saving treatments should be provided regardless of their cost. This contention is based on the Harvard philosopher John Rawls' theory of justice. We offer three rules to limit the use of cost-effectiveness approaches: efficiency assessments should not be deployed (i) when the choice is between an only treatment and no treatment, or to (ii) prioritise between different patients and patient groups. However a well considered cost efficiency calculation may have its place (iii) where a patient has a choice between two or more equally safe and effective treatments. We rebut potential objections to this analysis, and conclude that there has been a tendency to classify appeals for orphan treatments as a minority interest and in conflict with the aims of public health and society at large. Rawls' concept of societal justice shows that a distinction between the individual and society in this context is bogus. The funding of orphan therapies is as much a matter for public health as the funding of treatments for other conditions. Treatment must not be withheld on economic grounds.