The prevalence of sleep complaints and sleep disturbances was studied prospectively in 98 consecutive adult asthmatic patients (mean age 45 years, 46% men) attending an out-patient clinic by means of questionnaires and sleep diaries. The results were compared with those from an age- and sex-matched group of 226 healthy individuals. The most common sleep disturbances among the asthmatic patients were early morning awakening (51%), difficulty in maintaining sleep (DMS; 44%) and daytime sleepiness (44%). With decreasing asthma control (i.e. increased number of acute asthmatic attacks) there was an increase of DMS, nocturnal wakefulness, nocturnal breathing problems and bronchodilator inhalations at night. A decrease in estimated sleep time (P less than 0.05) and increase in nocturnal wakefulness (P less than 0.05) was seen with decreasing daytime FEV1--measured as percentage of the predicted value (%FEV1). There was also significant correlation between increasing age and decreasing %FEV1 (P less than 0.01). Among the 26 patients who were only taking one oral bronchodilator, no definite difference regarding sleep quality was found between those treated with theophylline and those taking an oral beta 2-agonist. The prevalence rates of DIS, DMS and daytime sleepiness were about twice as high among the asthmatic patients than in the healthy population. It is concluded that impaired quality of sleep, with disturbed sleep during the night, early morning awakenings and daytime sleepiness, is common among patients with bronchial asthma.