A simple method for the quantification of intestinal epithelial cell proliferation in humans was evaluated. Endoscopic biopsy specimens were stained and microdissected, and the number of mitoses per gland or crypt was determined, as was the area of these units. The technique could readily be applied to tissue from the stomach, small intestine colon, and rectum. The number of mitoses and the size of the proliferative compartments increased caudally from the stomach to the cecum in humans. There was a good correlation between area and mitoses per gland or per crypt (r = 0.89; P less than 0.001), confirming the general equivalence between proliferative rate and compartment size. The method was validated by comparing microdissection-derived data with data previously obtained as part of an autoradiography-based study in the dog. This showed that similar differences in proliferation and crypt cell mass could be obtained but in less than one sixth of the time taken to score autoradiographs. It is concluded that this method for the estimation of gastrointestinal epithelial proliferation correlates well with other well-established techniques, confers speed as well as accuracy, has an all encompassing denominator, and can thus avoid many of the problems associated with the quantification of tissue sections.