Desmoplastic cellular blue nevi.

We describe five cases of cellular blue nevus (CBN) accompanied by an abundant desmoplastic reaction. The cells within the desmoplastic regions reacted strongly immunohistochemically with antibodies to S-100 protein and HMB-45 and were negative to antibodies to smooth muscle actin and desmin, providing evidence that the abundant collagen deposits were the product of the nevus cells. The microscopic picture of desmoplastic CBN can differ considerably from that of common CBN, and differential diagnosis is discussed.