A blue naevus of the prostate: a light microscopic study including an investigation of S-100 protein positive cells in the normal and in the diseased gland.

A blue naevus of the prostate in an 80-year-old patient is presented, including electronmicroscopical examination and the demonstration of S-100 protein positivity. Cells expressing S-100 protein were sought in prostates of infants, young adults and elderly patients with hyperplasia and carcinoma. Positive staining was found in stromal cells, but the number diminished with age and disease. These findings could explain the rarity of blue naevus and the virtual absence of primary malignant melanoma in the prostate.