Eighteen adult subjects sensitive to mosquito bites participated in a double-blind, placebo-controlled study with 10 mg cetirizine. The drug was given prophylactically and the subjects were then exposed to bites of Aedes communis mosquitoes in the field. Bite lesions were measured and pruritus was scored with a visual analogue scale at 15 min, 60 min, 12 hr and 24 hr. Cetirizine significantly decreased immediate wealing and pruritus and, interestingly, also had a clear effect on the delayed 12 hr and 24 hr bite papules and pruritus. The diameter of a 15 min mosquito-bite weal was 10.10 ± 10.10 mm (mean ±s.d.) with the placebo and 5.5 ± 5.5 mm with cetirizine treatment (P<0.0). The 15 min pruritus scores were 36.36 ± 25.25 and 11.11±13.13 (P< 0.0), respectively. The diameter of the 24 hr mosquito-bite lesion was 12.12 ± 21.21 mm with the placebo and 7.7±16.16 mm with cetirizine treatment (P<0.0). The 24 hr pruritus scores were 18.18 ± 25.25 and 6.6±14.14 (P<0.0), respectively. These results indicate that, in mosquito-sensitive subjects, prophylactically administered cetirizine is an effective drug against both immediate and delayed mosquito-bite symptoms.