The three-dimensional (3-D) thickness of lamellae in normal trabecular bone was estimated by means of a stereologic transformation of the apparent width measured on 7 mm stained sections of undecalcified iliac bone from 65 nondiseased individuals aged 16--90 years. The average 3-D thickness of double lamellae (one bright and one dark) was 6.4 mm with a coefficient of variation between individuals of approximately 5%. Among the 32 males, the thickness was unchanged with age, while double lamellar thickness increased with age in the females [thickness (mm) = 5.980 (mm) ;.008 (mm/year) x age (yr). Intra-individual distributions of the thickness of double lamellae resembled normal distributions and had a median coefficient of variation of 23%. Within the trabecular osteons, no correlation between lamellar number and thickness was demonstrated, and no difference in thickness was found between mineralized and nonmineralized lamellae. Redeterminations showed no intra-observer bias in the estimation of the mean double lamellar thickness. The low variance between individuals indicated that the lamellar thickness is important for some essential biologic function, most likely the mechanical strength of trabecular bone. It may, therefore, be a useful variable to study in metabolic bone diseases. Furthermore, counting lamellae may be a simple way of estimating the 3-D thickness of remodeling sites in trabecular bone.