Acquired unilateral melanocytic nevi in otherwise normal skin

We describe a case with numerous melanocytic nevi in otherwise normal skin. A 5-year-old girl presented with more than 100 small pigment lesions on her left arm, shoulder and upper back without underlying light brown macule. The pigment lesions were first found on her left forearm at 3 months old and gradually increased along with her growth. Skin biopsy from a pigmented lesion shows a pathological change in compound-type melanocytic nevus without any atypical changes. Speckled lentiginous nevus is known to have multiple melanocytic lesions on the underlying brown macule from birth. Partial unilateral lentiginosis is characterized by unilateral lentigines with histopathological changes in lentigo but not melanocytic proliferation in the dermis. Agminated melanocytic nevi tend to be clustered together in a circumscribed area, whereas in the present case melanocytic nevi were segmentally arranged but not agminated. We consider that this is an unusual type of mosaicism of melanocytic disorders.