The nevoid pili multigemini over the back.

Pili multigemini is a rare developmental defect of hair follicles. It usually involves the beard of an adult and the scalp in children, but there have been no reported cases on the back. We report 3 cases of extensive pili multigemini over the entire back of healthy males. All cases were found incidentally. The direct microscopic examination of one hair follicle showed that it consisted of more than twenty shafts, which had rough surfaces and brush-like bulbs. The histological examination revealed that complicated follicular structures formed as many as 23 to even 36 hair shafts. Lots of hairs were surrounded by each inner root sheath and had one common outer root sheath. We suggest this nevoid is pili multigemini over the back.