Hypomelanosis of Ito (HMI) is a systemic neurocutaneous disorder characterized cutaneously by hypopigmented areas with irregular borders, streaks, whorls, or patches that are more evident on hyperpigmented skin or by examination under the ultraviolet light of a Wood lamp. These areas show a reduction in the number of melanocytes and in the size and number of melanosomes. Other cutaneous lesions such as cafe au lait spots, cutis marmorata, angiomatous nevi, heterochromia of iris or hair, and diffuse alopecia or trichorhexis may be present. More than 60% of cases also show clinical, radiologic, or histologic abnormalities of the central nervous system, with the most common abnormalities being impaired intellectual development and seizures (summary by Urgelles et al., 1996). There is evidence that the disorder is not a distinct entity but is rather a symptom of many different states of mosaicism (summary by Happle, 1998). See also linear and whorled nevoid hypermelanosis (LWNH; 614323), a benign pigmentation disorder with some similarities.