Does the Precautionary Principle have to be followed in every case where there's a potential injury to the environment or to public health? Is there a negative social price for following this principle? How do we determine what is more important – cost or benefit ? This was the main question faced by the Israeli Supreme Court in dealing with the disqualification of a water source by The Ministry of Health. The article deals with the development of the principle in international environmental law and its implementation in Israeli legislation and court rulings.