The morphology of the cardiac veins that drain into the coronary sinus was studied in 31 human hearts, with special reference to the coronary artery dominant pattern (Barnes' classification). The results were as follows: 1. Great cardiac vein (GCV) The average GCV index of Barnes type I (BI) did not differ significantly from that of Barnes type II (BII). 2. Left marginal vein (LMV) (1) The LMVs were divided into two types, according to the angle made by the LMV and the truncal vein: rect (18 cases, 58%) and acute (11 cases, 38%) angular influx type. (2) The average LMV angle of BI (n = 9, mean +/- SD 77.0 +/- 23.2) was greater than that of BII (n = 19, mean +/- SD 48.1 +/- 34.3) and the difference was statistically significant (p < 0.05). (3) The average LMV index of BI did not differ significantly from that of BII. 3. Left posterior ventricular vein (LPVV) (1) The average LPVV influx angle of BI did not differ significantly from that of BII. (2) The average LPVV index of BI did not differ significantly from that of BII. 4. Middle cardiac vein (MCV) The average MCV index of BI did not differ significantly from that of BII. 5. Small cardiac vein (SCV) The average SCV index of BI did not differ significantly from that of BII. 6. Anterior cardiac veins (ACVs) The number of ACVs correlated with the SCV index, and the correlation was statistically significant (n = 31, r = -0.416, p < 0.02). 7. Coronary sinus (CS) The average CS index of BI did not differ significantly from that of BII. The right ventricular wall may not be perfused effectively during open heart surgery when retrograde coronary sinus perfusion (RCSP) is used. The present study demonstrated that this method can produce poor right ventricular perfusion in cases when the ACVs developed well.