Acanthosis nigricans (AN) is a skin disorder characterized by abnormal darkening (hyperpigmentation) and 'velvety' thickening (hyperkeratosis) of the skin of the entire body, particularly in flexural areas, such as the neck and axillae. AN may be benign or associated with an underlying malignant pathology. The exact incidence of the benign form of AN is unknown. AN is much more common in people with darker skin pigmentation. The prevalence is between 1% and 5% in white people, and around 13% in Blacks. Obesity is closely associated with AN, and more than one half of the adults who are severely overweight have lesions consistent with AN. The incidence of AN is equal for men and women, and benign AN lesions may be present at any age, including at birth, although it is found more commonly in the adult population. Patients with the benign form of AN experience very few, if any, complications of their skin lesions, but most of them have an increased insulin level. Elevated levels of insulin probably cause AN, because they activate insulin receptors in the skin and induce tissue overgrowth. Endocrine disorders other than diabetes have been reported to be associated with AN: acromegaly, polycystic ovaries with hyperandrogenism and hirsutism, and hypothyroidism. A combination of AN with ectodermal defects (loss of the eyebrows or of the eyelashes) has been described in a single family with an autosomal dominant mode of inheritance. AN with insulin resistance, severe muscle cramps, large puffy hands, and enlarged kidneys was described in a brother and sister. The sister had polycystic ovaries with virilization. Following dilantin treatment, the cramps diminished dramatically and insulin resistance decreased in the male. In this family, this association of anomalies was probably transmitted by an autosomal recessive mode of inheritance. When benign and isolated, AN may be familial and inherited as an autosomal dominant trait. In association with insulin resistance it behavesas either a dominant or a recessive trait, and autosomal recessive inheritance may be observed when AN is part of Seip syndrome (see this term). It has been suggested that genes that influence AN have a pleiotropic effect on diabetes and its risk factors. AN may also be chemically-induced: nicotinic acid, diethylstilbestrol, oral contraceptives, and exogenous glucocorticoids have been suggested as risk factors. When AN develops in people who are not overweight, a medical check-up should be done to check for a tumor. The malignant forms of AN are far less common, occurring in about 1 in 6,000 patients with cancer. In these cases AN may be seen in additional locations, such as the lips or hands and is unusually severe. In contrast to the benign form, there is no racial propensity with malignant AN. Malignant AN often results in serious complications because the underlying malignancy is often an aggressive tumor. Malignant AN occurs more frequently in the elderly; however, cases have been reported in children with Wilms tumor. Average survival time of patients with signs of malignant AN is 2 years. Management of the benign form of AN involves reducing the circulating insulin by a simple diet, which may significantly help to resolve AN. Other treatments include tretinoin, 20% urea, alpha hydroxyacids, and salicylic acid.