The erythrocyte sedimentation rate (ESR) remains the most widely used laboratory test for monitoring the course of infections, inflammatory diseases, and some types of cancer. Several test methods have been developed recently, and as a result the safety and reliability of ESR testing procedures have improved. The method recommended by the International Council for Standardization in Haematology and the National Committee for Clinical Laboratory Standards for ESR measurement is based on the traditional Westergren method, using EDTA-anticoagulated samples without dilution. In clinical laboratories, reliable methods for calibration and the use of appropriate control materials are requiredfor monitoring the accuracy and precision of the routine method. We describe and evaluate a procedure for achieving the daily quality control of ESR and for establishing the limits of agreement between working and reference methods. Data from routine patient samples were used to calculate the daily cumulative mean and to monitor its reproducibility over time. Finally, to monitor analytic performance, a comparison was made between results from the measurement of ESR in specimens stored at 4 degrees C for 24 hours and results obtained in fresh samples.