The term hypomelanosis of Ito has been used as a diagnosis for individuals with hypopigmentation or depigmentation distributed along the lines of Blaschko. Approximately half of these patients have had neurologic, skeletal, and/or ocular abnormalities. In many, determination that the lighter areas of skin were hypopigmented rather than the darker areas hyperpigmented has been arbitrary. Evidence documenting single-gene transmission is unconvincing and recurrence risks appear to be negligible in most instances. Karyotyping of blood lymphocytes, skin fibroblasts, and/or keratinocytes of 115 individuals reported in the literature revealed abnormal chromosome constitutions in 60. Three patients were 46,XX/46,XY chimeras, two were 46XX/46,XX chimeras. Most patients were mosaic for aneuploidy or unbalanced translocations, with two or more chromosomally distinct cell lines either within the same tissue or between tissues. The more common alterations included mosaic trisomy 18, diploidy/triploidy, mosaicism for sex chromosome aneuploidy, and tetrasomy 12p. Karyotyping of blood and, if necessary, skin, to detect mosaicism is warranted in all patients presenting with swirley pigmentary changes, either hyperpigmentation or hypopigmentation. The terms hypomelanosis of Ito and incontinentia pigmenti achromians should be abandoned as they are neither diagnostic or specific.