The ruling by France's highest administrative court followed a complaint by the widow of a Jehovah's Witness who had received a blood transfusion against his explicit instructions.
The patient, who was not identified, later died. The court ruled that "whatever their obligation to respect the wishes of a patient based on that person's religious convictions, the doctors were not at fault."
The lawyer for the plaintiff said he was considering taking the issue to the European Court of Human Rights.