The difference in trabecular meshwork cellularity between fellow eyes was determined in a series of seven pairs of normal human eyes. Variations in cellularity between different quadrants of individual eyes also were determined. Characterization of these differences and development of appropriate sampling techniques to determine trabecular cellularity has become important in light of recent studies that find a decrease in trabecular cell numbers with age and with primary open-angle glaucoma, and the apparent stimulation of trabecular cell division by laser trabeculoplasty. The difference in absolute cell numbers (nuclei per section) between fellow eyes was 12.8% +/- 4.3% (standard error of the mean, SEM). The difference in cellularity (nuclei per solid tissue area) was 9.4% +/- 4.5% (SEM) in a subgroup of five pairs of these eyes. The coefficient of variation between different quadrants of an eye was 18.4% for absolute cell numbers and 17.3% for cellularity for the entire group of eyes. Sample size calculations indicate that for future studies, four pairs of eyes would be needed to detect a 20% difference in absolute cell numbers between fellow eyes and three pairs needed for detecting a similar difference in cellularity (alpha = 0.05, beta = 0.1). At least two quadrants per eye should be examined.