The major sources of error and variation in estimates of basement membrane thickness (BMT) are attributable to variations in conditions of specimen preparation, errors in magnification, and sectioning artifacts related to variation in section thickness and tangential sectioning. A simplified method for estimating BMT, requiring only two measurements per vessel, is described and justified theoretically from analysis of a model system. The chief merits of the method are: (1) the estimate of BMT obtained closely approximates true BMT, (2) variations in measurements related to section thickness and tangential sectioning are minimized, (3) coefficients of variation are smaller than for measurements obtained by averaging multiple measurements around the circumference of the vessel, and (4) a considerable reduction in time and effort required to obtain estimates of vascular BMT. Validation of these advantages is demonstrated in estimates of vascular BMT in a group of young adult subjects. The importance of correcting for asymmetrical distribution of basement membrane measurements prior to statistical analysis, of eliminating inhomogeneity in control groups, and of defining the normal range of basement membrane thickness statistically is discussed.