Fluid overload predisposes chronic hemodialysis patients to cardiovascular disease, a significant cause of morbidity and mortality in these patients. We evaluated the efficacy of monitoring changes in blood volume during routine hemodialysis to detect fluid overload. Intradialytic changes in blood volume were monitored by continuously measuring hematocrit in all 56 patients in a single dialysis unit over 7 weeks. After Week 1, patients were categorized into 2 separate groups depending on their maximum intradialytic decreases in blood volume. In Group 1, 46 of 56 or 82% had greater than a 5% decrease in blood volume while in Group 2, 10 of 56 or 18% had less than a 5% decrease in blood volume. During Weeks 2-7, dialytic fluid removal was intentionally increased in Group 2 patients by 0.80 +/- 0.62 L (mean +/- SD) or 47 +/- 43%. This intervention resulted in a larger (p < 0.02) intradialytic decrease in body weight (2.7 +/- 0.9 kg versus 2.0 +/- 0.8 kg) and a larger (p < 0.02) intradialytic decrease in blood volume (15 +/- 5% versus 4 +/- 1%) than experienced during Week 1 with a low incidence of symptoms. We conclude that there is a significant percentage of chronic hemodialysis patients who can tolerate additional fluid removal without hypovolemic symptoms even though they are considered to be at dry weight by routine physical examination and that the identification of these patients can be facilitated by intradialytic blood volume monitoring.