Over a seventeen years period we observed 5 patients presenting with very similar lesions the equivalent of which we could not find in the literature. These were pigmented and more or less atrophic bands which exactly followed Blaschko's lines. The lesions appeared during childhood or adolescence, between the ages of 6 and 20 years and always were unilateral. In all cases they were located on the trunk where they formed a recumbent "S" pattern characteristic of Blaschko's lines. They often started 3 to 6 cm away from the posterior midline and ended precisely on the anterior midline. These bands varied in number and in 3 out of 5 cases they were associated with pigmented atrophic bands or lines on the limbs of the same side. The intensity of pigmentation and atrophy was variable. The lesions were asymptomatic and only caused a cosmetic prejudice. They were perfectly fixed, and during a 2 to 30 years observation period they remained unmodified. The five skin biopsies performed on 3 patients showed no abnormality of the epidermis other than irregular and moderate hyperpigmentation of its deep part. In the dermis, there was no distinct pigment incontinence, no inflammation and no alteration of connective tissue texture; the elastic network was invariably normal. The impression of skin atrophy was not due to true dermal hypoplasia and perhaps corresponded to atrophy of the subcutaneous cellular tissue. No biological disturbance was observed. These lesions can easily be distinguished from epidermic naevi, incontinentia pigmenti and areas of hypoplastic dermis.(ABSTRACT TRUNCATED AT 250 WORDS)