An 8-year-old girl with atopic dermatitis presented with a 3-day history of a diffuse eruption of pruritic, umbilicated, erythematous vesicles with erosion and crusting on the flexor surfaces of the legs (Panel A) and arms. Her parents reported that eczematous lesions had previously occurred in these locations. The patient was afebrile, and no lymphadenopathy was present. A vesicle was unroofed, and cytologic analysis of the fluid revealed giant, ballooned keratinocytes that were consistent with herpes virus infection. A direct fluorescent antibody test was positive for herpes simplex virus (HSV) type 1. The patient received a diagnosis of eczema herpeticum, an eruption of a viral infection on a preexisting site of skin disease — typically, atopic dermatitis. Eczema herpeticum is most commonly caused by HSV. The differential diagnosis includes impetigo, bullous impetigo, scabies, eczema vaccinatum, and primary varicella infection. Bacterial superinfection is an important consideration. Eczema herpeticum can be severe and in some cases life-threatening. Early diagnosis and systemic antiviral treatment can minimize complications. The patient was treated with acyclovir, and the lesions healed completely after 3 weeks (Panel B).