The purpose of the paper is to analyse issues related to personal data processing within the framework of co-operation between the employer and trade unions in individual employment matters. The issues of personal data processing in collective relations are becoming more important especially due to lack of detailed legislative solutions, which results in the necessity to apply the general provisions of the GDPR as well as the latest changes to the Trade Unions Act. Pursuant to these changes, not only employees, but also persons performing gainful employment who are not employees can create and join trade unions as well as can be defended by the trade unions. The legislator frequently finds it difficult to adjust the protection of personal data to the specificity of collective relations, and the way in which the relationship between employers and trade unions is shaped in individual employment matters raises many questions in the context of personal data protection. The issue in question has not been explored in depth in the professional labour law literature. Thus, research on this subject is novel and justified from both the theoretical and the practical perspective. In the article, the formal-dogmatic method of research was used.