Rare diseases are a heterogenic group of disorders with a little in common except of their rarity affecting by less than 5 : 10.000 people. In the world is registered about 6000-8000 rare diseases with 6-8% suffering population only in the European Union. In spite of rarity, they represent an important medical and social problem due to their incidence. For many rare diseases have no treatment, but if it exists and if started on time as being available to patients, there is a good prognosis for them to be able for normal life. The problems of patients affected by rare diseases are related to the lack of diagnosis and timely undergoing as well as their treatment or prevention. Orphan drugs are products intended for treatment, diagnosis or prevention of rare diseases, but for their development and marketing the industry has not been interested in yet because of their marketing reasons. Patients suffering from a rare disease although belonging to the vulnerable group for their specific health needs, is becoming invisible in the health care system due to their additional needs un properly recognized. Ethical problems faced by patients, but also health care professionals are related to the allocation of medical diagnostics, unequal approach to health care, inappropriately specialized social services as well as therapy and rare orphan drugs unavailability. Ethical questions related to clinical trails on orphan drugs, population screening and epidemiology testing on rare diseases will also be discussed in this paper. [Projekat Ministarstva nauke Republike Srbije, br. 41004: Rare diseases: Molecular pathophysiology, the diagnostic and therapeutical modalities, social, ethical and legal aspects]