Sequential observations of coronary artery disease depend upon reproducibility of measurements of coronary dimensions. Because the coronary artery segment to be digitized for quantitation occupies only a small portion of the 35 mm cine frame, a cine projector was mounted on a movable X, Y stage so that the area of interest could be focused through a lens turret system onto a video camera. The lens turret provides lx, 3.5x, and 7x magnification. A digital image processing device operating under computer control permits direct application of smoothing and edge finding algorithms to the magnified coronary image. Using separately recorded grids, an external reference marker, and isocentering of the heart within a U-arm radiographic system, it is possible to adjust for distortion and magnification. A split screen technique provides capability for single plane or sequential biplane analyses. A digital lightpen is used to input fiducial points for segment identification and define an initial coronary margin which is superceded by computer defined edges. Final measurements include maximum, minimum and mean diameter of 1.0 cm long coronary segments and the absolute area of the segment profile, along with provision for hard copy image recording. Reproducibility of the minimum and maximum diameter measurements provided by the digitization system and edge finding algorithm are + .06 mm for vessels with an average diameter of 2.3 mm (+ S.D.). When duplicate coronary injections during the same catheterization procedure are made, the coefficient of variation of the minimum diameter is 9.5%, of the maximum diameter 5.0% and of the segment area 3.9%. The capability for area of interest selectivity, variable optical magnification and digital image analysis provides a unique and reproducible system for coronary segment quantitation.