<p>Accurate reticulocyte enumeration is essential for the diagnosis and management of anemia, directly influencing clinical decisions and patient care. This study aimed to develop and validate a national external quality assessment program for manual reticulocyte counting in Thailand. A pilot program was implemented at the Faculty of Associated Medical Sciences, Chiang Mai University, employing reticulocyte control materials prepared via new methylene blue staining, formaldehyde fixation, and mounting with synthetic resin. The control slides were rigorously evaluated for homogeneity and stability in accordance with international quality standards. Fifty clinical laboratories participated in two assessment cycles, each receiving duplicate samples for analysis. Performance was evaluated using robust statistical metrics and a standardized proficiency scoring system. The reticulocyte slides demonstrated excellent homogeneity and maintained stability during transport and storage. Results showed that more than 90% of laboratories engaged in both cycles, and all participants achieved excellent proficiency scores, reflecting a high standard of manual reticulocyte enumeration nationwide. Error analysis identified most issues as related to cell counting technique and clerical processes, which were addressed through focused technical training and process improvements. The implementation of this quality assessment scheme has strengthened laboratory capacity and fostered greater confidence in reticulocyte testing across Thailand. Continued participation and ongoing education are recommended to sustain high performance and ensure continued alignment with international best practices in laboratory hematology.</p>