The histologic features of prostatic duct‐acinar dysplasia have been difficult to analyze ultrastructurally, because of the difficulty in properly selecting and processing such small, randomly situated grossly invisible lesions. We have succeeded in identifying dysplastic foci by examination of the cut surfaces of tissue slices under low magnification. Dsyplasia foci were excised from the slices and were compared to adjacent normal tissue by both light and electron microscopy. By electron microscopy (EM), normal secretory cells were filled with myriad tiny clear vacuoles, which were markedly diminished to absent in the cytoplasm of dysplastic cells. Both apocrine and eccrine secretion characterized normal epithelium and were diminished in dysplasia. EM showed striking features of nuclear abnormality more prominently than light microscopy, and qualitative basement membrane abnormalities were revealed. By EM analysis, dysplastic epithelium resembled that of invasive carcinoma more than normal epithelial cells.