Circulating Blood Volume in Chronic Fatigue Syndrome David H. P. Streeten, MB, DPhil, FRCP, FACP David S. Be11, MD, FAAP ABSTRACT. Chronic fatigue syndrome (CFS) is an illness associated with severe activity limitation and a characteristic pattern of symptoms despite a relatively normal physical examination and routine laboratory evaluation. The recent description of delayed orthostatic hypotension in patients with CFS, and previous findings of reduced red blood cell (RBC) mass in other patients with orthostatic hypotension not known to have CFS, led us to measure RBC mass and plasma volume in 19 individuals (15 female, four male) with well characterized, severe CFS. RBC mass was found to be significantly reduced (p < 0.001) below the published normal range in the 16 women, being subnormal in 15 (93.8%) of them as well as in two of the four men. Plasma volume was subnormal in 10 (52.6%) patients and total blood volume was below normal in 12 (63.2%). The high prevalence and frequent severity of the low RBC mass suggest that this abnormality might contribute to the symptoms of CFS by reducing the oxygen-carrying power of the blood reaching the brain in many of these patients.