In Europe, a disease is considered to be rare when its prevalence is less than 5:10,000. With approximately 6000-7000 known rare diseases, 1 out of 20 individuals have such a disease. There is little interest in studying rare diseases because results of such research are only applicable to small groups of people. In addition, research into new drug development is considered not as relevant or financially lucrative as studies into more common diseases. We plead for a different manner of approaching rare diseases; namely, as a research model for human physiology and pathophysiology. The insights into pathophysiologic processes in rare diseases gained by research can subsequently be applied to more common diseases in which these same processes play a role. In addition, better collaboration between those involved in the field of science, such as patients and their support groups, academic hospitals and the pharmaceutical industry, is needed to stimulate research into rare diseases.