The frequency of asthma in 10 971 school-children between the ages of 5 and 14 years was reported by their parents to be 3-8%. Of these, 20-7% were said to have had bronchitis, 5-9% pneumonia, and 4-7% eczema. Asthma was reported more commonly in boys than girls and was greatest in children of social classes I and II. One-third of the children were reported to have their first attack before the age of 2 years. Few (18%) first attacks started after the age of 5 years. There was no evidence that bronchitis predisposed to the later development of asthma, or vice versa. Within each age-sex group children with a history of asthma had lower peak expiratory flow rates than children who gave no such history. These diffences in PEFR were greater than for children with a history of bronchitis.