Recirculation of blood flow occurs when the fistula flow rate is inadequate to support the desired dialyzer blood flow. The percentage recirculation is normally calculated using the blood urea nitrogen of blood samples from the two dialyzer blood lines and a peripheral blood sample. However, this method is time consuming, costly, and may not always give accurate measurements. A technique was developed to measure recirculation using the injection of saline into the venous dialysis line. For this technique, an optical detector is placed across the arterial dialysis tubing, and the light intensity, which is proportional to the hematocrit, is continually measured using a computerized data collection system. After a baseline data collection period, 10 ml of saline is injected into the venous dialysis line using the sampling port. The saline that appears in the arterial dialysis line as a result of recirculation will cause a dilution of the blood and an increase in light intensity. In vitro testing showed an excellent correlation between the area under the dilution curve and percentage recirculation. This technique will provide a quick, inexpensive, and reliable measurement of recirculation.