Pigmented lesions in the anogenital area encompass a wide variety of disorders including squamous intraepithelial neoplasia. The authors sought to explore the mechanism(s) underlying clinically pigmented squamous intraepithelial neoplasia in the anogenital area. A light-microscopic and immunohistochemical study of 64 lesional specimens from 45 patients (32 women, 13 men; age range, 23-73 years) with pigmented lesions in the anogenital area was performed. Histopathologically, 63 (98%) specimens showed melanin incontinence into the superficial dermis beneath the dysplastic epithelium. A focal or total loss of basilar hyperpigmentation was detected in 30 (48%) and 13 (20%) of lesions, respectively. In 17 (27%) cases, absence of basal layer hyperpigmentation was accompanied by a subepithelial lichenoid infiltrate. Melanin within the upper part of dysplastic areas were seen in 63 cases (98%), whereas dendritic melanocytes colonization, mild in all but 1 specimen case, was observed in 53 (83%) cases. All cases proved to be the usual type of squamous intraepithelial neoplasia; no single case of the simplex (differentiated) variant was present. The main mechanisms of pigmented squamous intraepithelial neoplasia of the anogenital area include melanin incontinence and occurrence of melanin in dysplastic keratinocytes. Colonization of the dysplastic epithelium by dendritic melanocytes seems to contribute, but it is rarely a prominent feature.