In short-term studies cetirizine effectively reduces the early and late phases of the cutaneous hypersensitivity reaction. The aim of this study was to determine its long-term effects on both the vascular and cellular components of the reaction. The skin blister technique was used to collect inflammatory cells after intradermal administration of grass pollen antigen to 10 atopic volunteers. They were treated for 3 months with 10 mg cetirizine twice daily. Tests were done at baseline, before, and 7, 30 and 90 days after initiation of treatment. Blister fluid containing cells was collected on microscope slides at 6 and 24 hours. The area of induration was measured at 0.25, 1, 6, 10 and 24 h. Cetirizine significantly reduced the peripheral blood eosinophil count at 30 and 90 days (75% and 40% reduction respectively); there was no significant change after only one week's therapy. Eosinophil recruitment to and activation in the area of antigen administration were already maximally reduced after 7 days, namely a reduction of 54, 52 and 59% at 10 h, and of 55, 68 and 66% at 24 h, respectively, at 7, 30 and 90 days. The area of induration was significantly reduced after one week of therapy. There was a general tendency towards an increase in the reduction at 30 and 90 days, which reached significance only at the 24 h observation; there was a 24, 51 and 48% reduction from baseline at, respectively, 7, 30 and 90 days. The data clearly show a progressive reduction of induration as well as of cellular events over time.(ABSTRACT TRUNCATED AT 250 WORDS)