In a double-blind randomized parallel-group study, six investigators enrolled 43 subjects to study the prophylactic effect of 10 mg cetirizine b.i.d. on grass pollen-induced asthma. The control group received 60 mg b.i.d. terfenadine, given to avoid withdrawals from the trial because of hay fever symptoms. Subjects were included in the study between the appearance of the first symptoms of hay fever and those of pollen-induced asthma. The hay fever and asthma symptoms, visual analogue scores (VAS), FEV1 and self-assessment data on complaints, salvage treatment and peak-flow values were statistically analysed. Both treatments were well tolerated, with a low and similar incidence of side-effects. During the last 3 weeks of treatment, six (32%) of the 19 subjects on cetirizine who were evaluable for efficacy remained free of asthma complaints, and another two (10%) had only a single minor attack. None had a grade 3 (incapacitating) attack. Conversely, only one (5%) of the 20 evaluable subjects on terfenadine remained complaint free, and all others (95%) had multiple attacks, which incapacitated three (15%) of them. Nasal obstruction, dyspnoea, morning peak flow, consumption of beta 2-mimetics and an efficacy index on asthma, combining complaints and rescue drug consumption, were significantly better on cetirizine (P less than 0.05). It is concluded that cetirizine is able to prevent the exacerbation of asthma induced by grass pollen.