To determine the frequency of nocturnal asthma in a non-hospital-based population, a survey was conducted of asthmatic patients being treated by primary care physicians in many parts of the United Kingdom. Seventy-four percent of the 7,729 patients who participated reported awakening at night at least once a week, and 64 percent reported awakening at least three times a week. Of 3,015 patients who regarded their asthma as mild, 26 percent reported awakening every night, suggesting that many patients underestimate the severity of their asthma. Patients with no nocturnal asthma generally had a lower frequency of allergic and nonallergic trigger factors, but no dominant feature distinguished these patients from those who did awaken at night. As perceived asthma severity increased, so did the number of drug types being prescribed, but no particular drug was identified as being associated with a lower frequency of nocturnal asthma. The frequency of nocturnal asthma found in the survey population (74 percent), in which 48 percent of the patients were using corticosteroid aerosols, was identical to that found in a survey conducted in 1971, when this medication was not yet available. Even though the sampling methods used in the two surveys were different, this finding indicates that the introduction of aerosol steroids has had little effect on the frequency of nocturnal asthma. Current use of existing medication alone may not eliminate the problem of nocturnal asthma, and new drugs and/or new dosing strategies may be needed to control this disabling symptom completely.