A congenital, hairy, giant pimentary nevus that covered the right half of the forehead of a newborn baby, was treated by Q-switched ruby laser. After 31 laser treatments of 8.0 to 9.0J/cm2 for 2 and half years, normally colored forehead skin with a few lanugos and small terminal hairs, developed. The hairs in the trea ed region were examined, and were found to be solitary rather than grouped, although two or three hairs were normally grouped in the scalp skin. The numbers of hairs were counted and compared with those in the untreated nevus. The number of hairs decreased to half of that in the untreated region and the nature also changed to vellus hairs or small thin terminal hairs. Histopathologically, all melanin-laden nevus cells and large terminal hairs disappeared from the dermis but a large number of melanin-unladen nevus cells was still immunohistochemically found in the dermis with anti-S 100 antibody. Moreover, slight fibrosis was detected in the dermis. Skin Research, 41: 613-620, 1999