Background: There are no published prospective, randomized, double-blind, placebo-controlled studies of urticaria prevention in children. Objective: Our objective was to study the effect of long-term treatment with the H 1 -receptor antagonist cetirizine in the prevention of urticaria in young children with atopic dermatitis. Methods: In the prospective, double-blind, parallel-group Early Treatment of the Atopic Child study, 817 children with atopic dermatitis who were 12 to 24 months of age at study entry were randomized to receive either cetirizine, 0.25 mg/kg, or matching placebo twice daily for 18 months and to be followed up for an additional 6 months, during which time the study medication code remained unbroken. During both these double-blind phases of the study, for a total of 24 months, caregivers prospectively recorded all symptoms and events, including hives, in a diary on a weekly basis when the child was well and on a daily basis when a symptom or event was observed. The diaries were reviewed and validated with the investigators at each regularly scheduled study visit. Results: Acute urticaria occurred in 16.2% of the placebo-treated children and in 5.8% of the children treated with cetirizine (P < .001). The protective effect of cetirizine disappeared when treatment was stopped. In the study population as a whole, urticaria episodes were most commonly associated with intercurrent infection or with food ingestion or direct skin contact. Conclusion: Acute urticaria is common in toddlers with atopic dermatitis and can be prevented with cetirizine in this high-risk population. (J Allergy Clin Immunol 2001;107:703-6.)