Normal and benign hyperplastic prostatic tissue was studied by quantitative electron microscopic measurements. Quantitative morphological procedures provide values for volume, surface, number of tissue and cellular components within human prostatic tissue. When a comparison is made of the stereological data of the glandular cell of benign prostatic hyperplasia to that of the normal human prostate no statistically significant difference in the relative volumetric amount of the cell organelles is indicated. An attempt was made to characterize the fine structure of the smooth muscle cells of the stromal area (fibromuscular) in normal and benign hyperplastic prostatic tissue. In benign prostatic hyperplasia a significant increase in the relative amount of organelles in the smooth muscle cell was found, indicating an activation of these cells. Light microscopic analysis has revealed that benign prostatic hyperplasia is primarily a stromal disease.