Although intraepithelial pigment in the prostate gland has been termed melanosis, the nature of the pigment is not entirely clear, and many pathologists are not aware of its existence. We examined 863 hematoxylin and eosin (H + E) stained slides from 150 surgical specimens of prostate (69 needle biopsies, 66 transurethral resections, 14 radical prostatectomies, and 1 suprapubic prostatectomy) from 149 patients (age range, 47 to 90 years; mean 70 years) in an effort to characterize this pigment. The 1-3 microns in diameter, predominantly subnuclear, yellow-brown to gray-brown granules with a dark blue rim (by H + E) stained positively with Fontana-Masson, periodic acid-Schiff with diastase, Congo red, luxol fast blue, and oil-red-O and exhibited yellow autofluorescence consistent with lipofuscin. H + E stained slides revealed pigment in the benign epithelium in 86 of 150 cases (57%), within stromal macrophages in eight cases, and in atypical epithelium in two cases of high-grade prostatic intraepithelial neoplasia. Ten cases of invasive adenocarcinoma without recognizable pigment in H + E stained sections were stained by the Fontana-Masson technique, and pigment was identified in malignant epithelium in three of these cases. Ultrastructural examination of intraepithelial pigment in KII-fixed tissue from three radical prostatectomy specimens demonstrated the typical appearance of lipofuscin. Although intraepithelial pigment in prostatic biopsy or resection specimens is usually considered characteristic of seminal vesicle epithelium, our study demonstrates that lipofuscin is commonly present in epithelial cells of benign prostatic hyperplasia and less frequently in those of prostatic intraepithelial neoplasia and adenocarcinoma. The recognition of this pigment is important in preventing diagnostic confusion with seminal vesicle epithelium and with melanocytic lesions.