The hallmark of incontinentia pigmenti (Bloch-Sulzberger syndrome) in the neonatal period is the presence of an erythematous vesicular rash. The rash may be present at birth or may develop shortly after, usually during the first 2 weeks of life. The vesicles usually form a linear pattern following the Blaschko lines but isolated lesions with no particular pattern may also occur (Figure 298.1). The evolutive changes that characterize incontinentia pigmenti (verrucous eruption and hypopigmented lesions) do not usually occur in the neonatal period. The most important entity to differentiate from incontinentia pigmenti is herpes simplex encephalitis. They both may produce seizures and have skin vesicles. The distinction is based on the location and cytology of the vesicles. The vesicles in herpes simplex infection tend to occur on the scalp or presenting body part (sites of trauma) and the scrapings from the base of the vesicles show multinucleated giant cells with intranuclear inclusion (Sank smear). The vesicles in incontinentia pigmenti tend to occur on the limbs or lateral trunk, and the scrapings from the base of the vesicles show large numbers of eosinophils. Neonates with incontinentia pigmenti have leukocytosis and a high blood eosinophils count.