The authors compared delivered and prescribed blood flow (QB) during 208 hemodialysis treatments using the Transonic hemodialysis monitor (Transonic Systems, Inc, Ithaca, New York). Delivered QB averaged 205.6, 300.6, 384.3 (p < .0001), and 467.7 cc/min (p < .0001) at pump settings of 200, 300, 400, and 500 cc/min. Permcaths had significantly lower delivered QB (188, 266, and 314 cc/min at 200, 300, and 400 pump settings) (p < .01). When catheters were excluded, delivered QB was at least 10% less than prescribed in 13.5%, 18.4%, and 20.8% of patients (300, 400, and 500 pump settings). Patients using 14 gauge arterial needles had higher delivered QB than patients using 15 gauge needles (418 vs 383 cc/min at 400 pump; p < .0001). Patients on Baxter 550 machines had lower delivered QB (294 and 376 ml/min) than those on fresenius 2008H machines (323 and 411 ml/min) at 300 and 400 pump settings, respectively (p < .0001). Excluding catheters, 17.4% and 23.4% of patients on Baxter machines had delivered QB of 10% less than prescribed, compared to 2.1% and 4.3% of patients on Fresenius machines at 300 and 400 pump settings. In conclusion, delivered QB during hemodialysis is often significantly less than prescribed QB. Further studies are necessary to determine the factors involved in these differences.