Erythema toxicum neonatorum is a common, non-threatening rash in newborns. It appears in 4-70% of newborns within the first week of life, and it typically improves within 1–2 weeks. It only occurs during the newborn period, but may appear slightly later in premature babies. The rash has a variable appearance. It typically includes blotchy red spots, often with overlying firm, yellow-white bumps or pus-filled boils. There may be only a few or many lesions. The lesions can appear almost anywhere on the body, and individual lesions may appear and disappear within hours. There are no other symptoms associated with erythema toxicum neonatorum, and the rash does not have any long-term effects on the skin. Erythema toxicum neonatorum is not harmful and does not require any treatment. Epidemiology The exact prevalence of erythema toxicum neonatorum is unknown, and studies estimate prevalence as low as 3.7 percent to as high as 72 percent. It is one of the most commonly diagnosed rashes in healthy babies. It is more common among infants born at higher gestational age and is rare among premature infants. Erythema toxicum neonatorum is more likely to develop in infants delivered vaginally. Higher birth weight is an additional risk factor. There may be a slightly increased risk in males, but this association is unclear. There are no known associations with race or ethnicity. Presentation Erythema toxicum neonatorum usually appears during the first week of life, most often on day two. It may develop several days or weeks later in premature babies. The rash has a variable appearance, ranging from a few blotchy red spots to many yellow-white bumps and boils. The classic presentation is 1–3 mm, firm, yellow-white bumps with a surrounding red halo. The rash is often described as "flea-bitten." There may be only a few or many lesions, and they may be clustered or widespread. The rash often appears on the cheeks first and may later spread throughout the face, trunk, arms, and legs. Lesions most often appear on the thighs, buttocks, and trunk. The palms and soles are not affected, likely because the skin in these areas does not have hair follicles. Individual lesions may grow and shrink over hours or days. They usually fade within a week or two. The rash may recur within the next few weeks, but this is rare. Cause Erythema toxicum neonatorum is related to activation of the immune system, but its