We studied 27 patients (seven male, 20 female) with stable mild-to-moderate asthma to measure their level of physical fitness and to determine if a relationship existed between aerobic fitness and the degree of airway reactivity, expiratory flow rates, or the amount of habitual leisure-time physical activity. Nonspecific bronchial hyperreactivity (NSBHR) was quantified by methacholine inhalation challenge. On a separate day, exercise capacity was evaluated with incremental exercise testing to exhaustion after bronchodilator pretreatment. The level of physical activity was assessed with a validated written questionnaire. FEV, was 78 +/- 13% predicted prebronchodilator and 92 +/- 14% predicted postbronchodilator. The mean provoking concentration of methacholine that caused a 20% decrease in FEV1 (PC20) was 1.14 +/- 1.38 mg/ml and ranged from 0.019 to 5.71 mg/ml. There was no correlation between PC20 and prebronchodilator FEV1 r = 0.37, p greater than 0.05). Mean maximal oxygen uptake (VO2max) was not significantly different from predicted normal values (36.9 +/- 10.8 versus 38.5 +/- 5.3, p = 0.32). Mean maximal O2pulse (maximal heart rate/VO2max), anaerobic threshold, and dyspnea index were within normal limits. There was no relationship between VO2max and FEV1 when expressed as percentages of predicted values (r = 0.08, p = 0.71) or between VO2max and PC20 (r = 0.23, p = 0.25). There was, however, a significant relationship between VO2max and the level of habitual leisure-time activity (F = 3.64, p less than 0.05). Results from the exercise questionnaire suggested that asthmatics perceive their disease as a limiting factor to improved aerobic fitness and that they lack adequate knowledge about asthma and exercise.(ABSTRACT TRUNCATED AT 250 WORDS)