We investigated whether videodensitometry provides a possibility of objective plaque characterization by intracoronary ultrasound imaging. Histologically matched 30 MHz ultrasound images of characteristic atherosclerotic lesions in saline solution and blood-perfused human coronary arteries in vitro were digitized, and regions of interest were analyzed by calibrated videodensitometry. Pixel gray-level distributions were represented as frequency histograms, and mean pixel gray level, skewness, and kurtosis were calculated. Similarly, images of flowing blood in vitro (n = 9; hematocrit 40%) were analyzed. Mean pixel gray levels differed between the three lesion types (lipid rich [n = 6], 71 to 92 [mean 80]; fibromuscular plaque [n = 14], 94 to 162 [mean 124]; and calcified plaques [n = 8]; 161 to 196 [mean 178]) (all comparisons p < 0.001). Ranges of mean pixel gray levels for flowing blood in vitro were within the range of those for fibromuscular plaque (86 to 103; mean 98). During blood perfusion in nine specimens, pixel gray levels were 85% +/- 7% of those during saline perfusion but with similar differences between plaque types. We conclude that homogeneous plaque types can be distinguished in vitro by videodensitometry on intracoronary ultrasound images.