Objectives: Our study aimed to determine the frequency and type of non-conformities that occurred in the tissue typing laboratory (TTL), Rijeka, Croatia, in order to evaluate the quality of testing and compliance with the requirements of the quality management system (QMS). Background: The QMS in a TTL should lead to improvements in the accuracy and timeliness of results. One of its essential elements is non- conformity management. Methods: A retrospective analysis of non- conformities recorded in the TTL Rijeka from 2006 to 2017 revealed the overall frequency of 0·43%. Classification was performed according to the examination phase, employee characteristics, relation to the field of work, part of laboratory and priority of testing. Results: The most reported non-conformities occurred during the examination phase (54·0%), followed by the pre- and post-examination phase (31·2 and 14·8%, respectively). Inadequate competency and less experience were associated with the number of reported non-conformities while no difference was observed in relation to the profession or position in the laboratory. Non-conformities were equally distributed between organ transplantation and HLA-associated disease diagnostic processes, mostly referring to the entire laboratory work. Conclusion: Intensifying training and education, implementation of information technology into laboratory processes, more rigorous error detection and reporting are fundamental steps in improving quality of laboratory services.