Blood hyperviscosity due to secondary erythrocytosis is a common pathologic feature of cyanotic congenital heart disease (CCHD). In CCHD, it is possible that hematological parameters other than red blood cells influence blood rheology. We measured blood passage time to evaluate the blood rheology in patients with CCHD (n=18, age: 15.3+/-11.9 years, mean +/- SD) and age-matched control subjects (n=27) using the microchannel array flow analyzer (MC-FAN), and the results [several hematological parameters, including hematocrit (Hct)] were compared. Blood passage time in the CCHD group was prolonged, compared with the control group (67.6+/-27.2 s vs. 44.6+/-6.7 s). For the CCHD group, blood passage time correlated significantly with red blood cell (RBC) count, hemoglobin (Hb) concentration, Hct, mean corpuscular hemoglobin concentration (MCHC), platelet (Plt) count, high-density lipoprotein cholesterol (HDL-C) level, and triglycerides (TG) level (RBC, r=0.77; Hb, r=0.69; Hct, r=0.73; MCHC, r=-0.64; Plt, r=-0.49; TG, r=0.53; HDL-C, r=-0.49, p<0.05 for each variable). For all 45 subjects, blood passage time correlated significantly with HbA1c level (r=0.45, p<0.01) and tissue-type plasminogen activator (t-PA) antigen level (r=0.46, p<0.01). Our results indicated that blood rheology is reduced in patients with CCHD as expressed by prolonged blood passage time, and it may be defined by several blood parameters in addition to erythrocytosis.