We conducted a 20-year prospective review of 41 pediatric patients with a diagnosis of hypomelanosis of Ito. No evidence pointed to hereditary transmission of the disease. Associated extracutaneous pathology was mainly of the nervous and musculoskeletal systems. Three of 19 patients had chromosomal anomalies. Hypomelanosis of Ito is a clinically well-characterized syndrome in which chromosomal instability may be a component. Criteria for its presumptive and definitive diagnosis are proposed.