Objective  To compare the two stereological method  based on profile counts and diameters for the estimation of renal tubular length.   Methods  Nine kidneys from 2-month-old normal male SD rats were obtained and isotropic random sections (methacrylate resin-embedded) were prepared with the orientator for stereological measurement after staining with periodic acid-Schiff (PAS) reagent and hematoxylin. The length of renal tubule segments (including collecting ducts, not including the thin segments of Henle’s loops) was estimated by counting the number of central lumens of renal tubular profiles on sections. Approximate round or elliptical renal tubular profiles with an apparent lumen were sampled and their diameters (round profiles) or short diameters (elliptical profiles) were measured; from the mean of squared diameters, the cross-sectional area of renal tubules (assumed as cylindrical structures) was calculated for the length estimation in combination with the volume of renal tubules.   Results  The ratio of the total length of renal tubules per kidney as estimated with profile diameters to that estimated with profile counting was 1.43 on average (coefficient of variation 7.2%), i.e. the method  with profile diameters overestimated the renal tubular length by more than 40%.   Conclusion  The method  of estimating renal tubule length with the short diameters is not reliable.