Asthmatic patients hyperventilate during acute attacks, but controversy persists as to whether they breathe rapidly, deeply or both. We monitored respiratory rate under the three following conditions: (1) asthma treated in the emergency room; (2) airways obstruction provoked by methacholine inhalation; and (3) airways obstruction provoked by exercise. In 47 acutely ill asthmatic patients, respiratory rate was higher than in 42 nonasthmatic control patients in the emergency room. Pretreatment respiratory rate correlated with peak expiratory flow rate and forced expired volume in one second. In 17 asthmatic patients and 16 healthy volunteers, breathing pattern was monitored by respiratory inductance plethysmography. Methacholine inhalation and exercise provoked significant airways obstruction in asthmatic patients but not in control subjects. In asthmatic patients, minute ventilation and tidal volume increased above that of control subjects following methacholine and exercise, but the rate was no higher than in control subjects. We conclude that the respiratory rate is increased in naturally occurring asthma, but not when acute airways obstruction is induced transiently in the laboratory. In the former setting, the respiratory rate is correlated with spirometric measures of airflow obstruction, but the weakness of the correlation does not allow the respiratory rate to be used as a substitute for spirometry.