Studies evaluating newer antihistamines in children are few. Levocetirizine is a potent and highly selective H1-antihistamine with a proven efficacy in adults. Primary objective was to assess the efficacy of levocetirizine 5 mg once-daily in reducing seasonal allergic rhinitis (SAR) symptoms, as measured by Total Four Symptom Score (T4SS = sum of sneezing, rhinorrhea, nasal and ocular pruritus), over the first 2 wk of treatment. Efficacy over 4 and 6 wk of treatment, effect on nasal congestion and on health-related quality of life as measured by PRQLQ (Paediatric Rhinoconjunctivitis Quality of Life Questionnaire) were among the major secondary objectives. A double-blind, randomized, placebo-controlled study including 177 children with a documented SAR (to grass and/or weed) for at least a year and having a mean baseline T4SS ≥ 6 (out of 12). Children evaluated daily the severity of T4SS and nasal congestion on a scale from 0 (absent) to 3 (severe). PRQLQ responses were assessed on a scale from 0 (not bothered) to 6 (extremely bothered) and analysed descriptively. Global evaluation of disease evolution judged by investigators, parents and children was made on a scale from 1 (marked worsening) to 7 (marked improvement). For the primary objective, levocetirizine was statistically highly superior to placebo with a difference in adjusted means of 1.29 (95% CI: 0.66–1.92) in favour of levocetirizine (p < 0.001). The effect of levocetirizine was almost twice that of placebo (94.1% relative improvement over placebo). Nasal congestion was improved with levocetirizine reaching maximum difference to placebo of 0.31 (p < 0.05), a relative improvement over placebo of 77.5%. PRQLQ scores at week 2 improved with levocetirizine more than with placebo (0.85 vs. 0.51, respectively) remaining larger after 4 and 6 wk of treatment. In the study, 84.3%, 80.9%, 80.9% of children had their disease evolution rated as slightly-to-markedly improved by, respectively, the investigators, the parents and children themselves. Incidence of treatment-emergent adverse events was similar in both groups (33.7% with levocetirizine; 30.7% with placebo). No child in the levocetirizine group discontinued treatment because of adverse events. The 6-wk duration of this study was longer than the usual 2–4-wk duration for similar studies and shows that levocetirizine controls SAR symptoms in children over the entire pollen season.