An acne-free naevus.

Investigation of rare disorders may sometimes help in the understanding of common problems. We describe a patient with an unusual and previously unreported acne-free naevus occurring in an area severely affected by acne. Investigation revealed smaller sebaceous glands, a reduced sebum excretion rate, decreased number of Corynebacterium acnes in the pilosebaceous ducts and a reduced conversion of testosterone to 5 alphadihydrotestosterone in the areas which were free of acne compared with acne-bearing skin. The results of these investigations support some of the suggested mechanisms responsible for the development of acne.