During angioplasty of artificial stenoses, atherosclerotic human cadaver arteries, and normal canine arteries, pressure and volume of the dilatation balloons were continuously recorded. We found that a sudden yield of a lesion corresponded to a sudden drop in the pressure curve and an increase of the balloon volume. Volume monitoring was insensitive, but pressure recording was very precise. Continuous pressure recording, using a non-compliant inflation system, correctly demonstrated small breaks and ruptures of atherosclerotic vessels, changes not seen on fluoroscopy.