The Thrombolysis In Myocardial Infarction (TIMI) frame count is a relative index of coronary flow that measures time by counting the number of frames required for dye to travel from the ostium to a standardized coronary landmark in a cineangiogram filmed at a known speed (frames/s). We describe a new method to measure distance along arteries so that absolute velocity (length divided by time) and absolute flow (area x velocity) may be calculated in patients undergoing percutaneous transluminal coronary angiography (PTCA). After PTCA, the guidewire tip is placed at the coronary landmark and a Kelly clamp is placed on the guidewire where it exits the Y-adapter. The guidewire tip is then withdrawn to the catheter tip and a second Kelly clamp is placed on the wire where it exits the Y-adapter. The distance between the 2 Kelly clamps outside the body is the distance between the catheter tip and the anatomic landmark inside the body. Velocity (cm/s) may be calculated as this distance (cm) divided by TIMI frame count (frames) x film frame speed (frames/s). Flow (ml/s) may be calculated by multiplying this velocity (cm/s) and the mean cross-sectional lumen area (cm2) along the length of the artery to the TIMI landmark. In 30 patients, velocity increased from 13.9 +/- 8.5 cm/s before to 22.8 +/- 9.3 cm/s after PTCA (p <0.001). Despite TIMI grade 3 flow both before and after PTCA in 18 patients, velocity actually increased 38%, from 17.0 +/- 5.4 to 23.5 +/- 9.0 cm/s (p = 0.01). For all 30 patients, flow doubled from 0.6 +/- 0.4 ml/s before to 1.2 +/- 0.6 ml/s after PTCA (p <0.001). In the 18 patients with TIMI grade 3 flow both before and after PTCA, flow increased 86%, from 0.7 +/- 0.3 to 1.3 +/- 0.6 ml/s (p = 0.001). Distance along coronary arteries (length) can be simply measured using a PTCA guidewire. This length may be combined with the TIMI frame count to calculate measures of absolute velocity and flow that are sensitive to changes in perfusion. TIMI grade 3 flow is composed of a range of velocities and flows.