The feasibility of moderately severe airway hyperresponsiveness (AH) was examined as an inclusion criterion for clinical trials in asthmatic children. During the baseline period of a long-term clinical trial in asthmatic children, maintenance therapy with fluticasone (200 microg x day(-1)) was stopped for a maximum of 8 weeks and methacholine challenges were performed at 2-week intervals or earlier if the patients' condition deteriorated. Patients were eligible to continue the study if the provocative dose of methacholine causing a 20% fall in forced expired volume in one second (FEV1) (PD20) was <80 microg. Fifty-one per cent of the children did not develop a PD20 < 80 microg after withdrawal of fluticasone. Patients with or without a PD20 <80 microg did not differ in duration of asthma, duration of treatment, or peak flow variation. Patients with a PD20 <80 microg had higher levels of total and specific immunoglobulin-E, and lower levels of FEV1 and mean maximal expiratory flow than patients with a PD20 > or = 80 microg. Forty-four per cent of the patients with a PD20 > or = 80 microg did not have any symptoms during the wash-out period and 39% of these patients remained free from symptoms during one year follow-up. The results of this study suggest that recruiting asthmatic children for clinical trials may be difficult if airways hyperresponsiveness is used as the sole inclusion criterion.