Mandatory Disclosure Rules (MDR) have been introduced to the Polish legal system and have been in force since 1 January 2019. It is a regulation imposing an obligation of disclosing tax planning schemes to fiscal administration bodies, and its purpose is to protect the tax system against tax evasion and dishonest lowering of public levies. This regulation introduced significant changes in the scope of practising the profession of a tax advisor with a particular consideration of a change made in the professional secrets of a tax advisor. These changes gave rise to many controversies and doubts both in the doctrine as well as among the tax advisors’ self-governing council. The newly-introduced institution may have a negative impact on the practical aspects of performing the profession of tax advisor. The aim of the work is to discuss and analyse the Mandatory Disclosure Rules in terms of professional confidentiality of a tax advisor, as well as to indicate the practical effects of the introduced amendment. The scientific article uses the formal-dogmatic method, analysing the legal text and its interpretation, including jurisprudence.