Airway hyperreactivity has been proposed to be an important determinant of severity of asthma and medication needs to control symptoms in adults. In this study we tried to determine if this relationship existed in childhood asthma. One hundred and forty-five asthmatic children aged 6 to 19 years with a positive methacholine (MCH) challenge test and a baseline forced expiratory volume in one second (FEV1) of 56% to 118% predicted were studied. The MCH concentration required to decrease the FEV1 from baseline by 20% (PC20) ranged from 0.1 to 20 mg/mL (geometric mean = 1.85 mg/mL). Asthma symptoms in this population before the study ranged from 2 months to 14 years. They were followed for a mean of 10 months after the MCH challenge and then grouped into four groups according to overall severity of symptoms and treatment needed to control symptoms. The first grade was comprised of patients with intermittent symptoms only, with a respiratory tract infection (URTI), and no medication; grade 2 symptoms were severe enough to require intermittent bronchodilators (BD); grade 3 symptoms were severe enough to require daily BD; and grade 4 symptoms were severe enough to require daily BD and steroids. Geometric means PC20 were significantly different among the four groups when they were analyzed by ANOVA P less than .01. There was, however, marked overlap between the individual levels of PC20 among the four groups. There was no significant difference in mean FEV1, age, sex, or duration of symptoms among the four groups. There was no significant correlation between baseline FEV1 and the degree of airway hyperreactivity.(ABSTRACT TRUNCATED AT 250 WORDS)