The present study seeks to estimate the difference between coronary zero flow pressure (Pzf) by analysis of the baseline pressure - flow relationship and the Pzf calculated during a long diastole in humans. Although Pzf is likely to provide meaningful information about the characteristics of coronary circulation, there are no available data on Pzf in humans because Pzf is overestimated when it is calculated during normal cardiac cycles. Actual Pzf was determined in 15 subjects by analyzing the coronary pressure - flow relationship during a long cardiac cycle induced by an intracoronary adenosine triphosphate (ATP) infusion, and it was compared with the Pzf calculated during a normal cardiac cycle in order to estimate the difference. Pzf calculated during a normal cardiac cycle was 47±15 mmHg, which decreased to 36±9 mmHg after intracoronary administration of ATP (0.05 mg) whereas actual Pzf was 21±7 mmHg. Pzf calculated in a pressure - flow relationship during a normal cardiac cycle under vasodilation correlated well with that during a long diastole (r=0.75, p<0.01), although it was 15±6 mmHg greater than the actual Pzf. It was concluded that Pzf during a normal cardiac cycle could be used to anticipate Pzf. (Jpn Circ J 2001; 65: 793 - 796)