Continuous measurement of blood flow velocity during interventional procedures has the potential to provide an early warning of coronary flow instability, which can lead to abrupt closure or other adverse events before angiography. The magnitude and fluctuations of the average velocity over time (trend) was studied by using a 0.018-inch Doppler-tipped angioplasty guide wire in 32 patients after coronary angioplasty (n = 20), atherectomy (n = 2), urgent stent (n = 6), urgent vein graft thrombolysis (n = 4), or acute myocardial infarction (n = 2). The patients (mean age 60 ± 11 years) had postprocedural in-laboratory flow monitoring for a mean of 19 ± 11 (range 8 to 36) minutes. The coronary artery monitored was the left anterior descending in 13, circumflex in 6, right coronary artery in 9, and saphenous vein graft in 4. Seven patients had flow-related events during continuous flow velocity monitoring before serial angiographic study. These events included coronary vasospasm (abrupt flow acceleration), vasovagal flow cessation, cyclical flow variations resulting from accumulation of intraluminal thrombus, and rapid decline of flow velocity. The last two patterns were associated with abrupt vessel closure during angioplasty. Continuous flow velocity monitoring is easily incorporated into routine interventional procedures and provides an early indication of unstable flow and the potential for abrupt vessel closure and other adverse events.