Registries in the area of renal replacement therapy (RRT) are intended to be a tool for epidemiological research, health care planning and improvement of quality of care. In this perspective, the value of a population-based RRT registry and its ability to achieve its goals rely heavily on the quality of its data and on quality control procedures. Data quality is of crucial importance to prevent ‘garbage coming out from garbage going in’. Regular evaluations of the data quality within these registries are needed to assess various aspects of data quality like completeness, prevention of duplicates, data validity, comparability, timeliness, usefulness and responsiveness of different items and accuracy of reporting and interpretation. In this technical note, we will discuss the purposes of RRT registries and the importance of different aspects of data quality in relation to those purposes. In addition, we discuss how these data quality aspects can be assessed and, where possible, how data quality may be improved.