Histological preparations of the rectal mucosa were analyzed quantitatively in 51 patients with chronic ulcerative colitis. Four histological variables were recorded: the diameter (minor axis) of the lumen of 10 consecutive (transversally cut) glands, the interglandular linear distance between 10 consecutive glands, the number of glands at high power field examination, and the number of nuclei in 10 consecutive areas of lamina propria. The most important feature found to differentiate colitic from noncolitic patients was the diameter of the lumen of the rectal glands. The sum of the values of the four variables has demonstrated that 94% of the colitic patients had scores of 22 or more. Previous work showed that only 0.8% of 124 noncolitic patients had a similar score.