The effect of cetirizine, 10 mg at night, on dermographic urticaria, was studied in 19 patients. The study design was a randomized, double-blind, crossover comparison with placebo, each treatment being given for 7 days. Patients kept a daily diary of itch and weal severity (100-mm linear analogue scale), and recorded sleep disturbance. The dermographic weal response was measured objectively with a spring-loaded stylus, and the weal threshold calculated from the force/response curve. There was a small, insignificant subjective response to placebo, but no objective response. On cetirizine, the subjective assessment of wealing was reduced from 34.3±6.7 (mean ± SEM, 0–100 scale) to 16.8±4.1 (P= 0.02), itch was reduced from 43.2±6.6 to 19.4±4.1 (P=0.001), and nights disturbed from 46.2 to 8.8% (P=0.03). There was a shift to the right in the position of the force/response curve, and the wealing threshold increased from 24.6±3.2 to 54.7±4.4 g/mm2 (P=0.00001), but there was no correlation between change in itch scores and wealing threshold. Cetirizine 10 mg daily is an effective treatment in dermographic urticaria, and its usefulness will depend on the prevalence of unwanted effects.