The impact of the prolonged use of cetirizine at high dose (0.25 mg/kg twice a day over 18 mo) on behavior and cognitive ability was examined in a double-blind, randomized, placebocontrolled trial (ETAC-Early Treatment of the Atopic Child) designed to establish whether it was possible to prevent young children (1-2 y old at study entry) with atopic dermatitis from developing asthma. Well-validated and standardized measures of behavior (Behavior Screening Questionnaire) and cognition (McCarthy Scales of Children's Abilities) were used. In addition, the ages of attainment of psychomotor milestones were established. These measures were taken between an average of 32 and 53 mo of age, both during the study treatment with cetirizine or placebo and after the study treatment had been discontinued. The Behavior Screening Questionnaire was completed at least once on approximately 300 children in each group and on approximately 200 children on five occasions. The McCarthy Scales of Children's Abilities were administered to approximately 100 in each group at three different times. There were no significant differences between the cetirizine and placebo groups on either of the behavior and cognition measures or in psychomotor milestones during or after the study treatment. These findings suggest that there are no adverse effects on behavior or learning processes associated with the prolonged use of cetirizine in young children with atopic dermatitis. There is evidence that the use of some antihistamines has an effect on behavior and that this is shown most markedly as sedation (1-3). These side effects may have implications for the long-term development of the child. Little is known about the development of histamine receptors with age, and, given that all antihistamines can gain access to the brain (4), it is important to establish whether antihistamines have a deleterious effect on histamine receptor sites and on the cognitive and behavioral systems regulated by histamine (5). There has been no study on the impact of the long-term administration of antihistamines on children's behavioral development. The present study aimed to fill this gap.A number of findings that have suggested that firstgeneration antihistamines can interfere with learning. Sedating (diphenhydramine) and nonsedating (loratadine) antihistamines and a placebo have been used with children suffering from seasonal allergic rhinitis (6). It was found that those on placebo and diphenhydramine learned significantly less well than healthy controls. The nonsedating antihistamine only partially counteracted this effect. A subsequent study of young adults with seasonal allergic rhinitis showed that this learning deficit could be avoided by the use of a combination compound of acrivastine and pseudoephedrine (7). The indications from these previous studies made it essential to determine whether