The hypothesis that bronchial asthma might follow a biorhythmic pattern was tested in 25 asthmatics with moderate to severe obstruction who completed daily diaries of respiratory symptoms and medication use. Biorhythm cycles (for 23-, 28- and 33-day periods) were calculated. Analysis of scores of medication use or respiratory symptomatology failed to demonstrate any relationship between asthma and biorhythm cycles or any other periodicity.