In a cohort of 1654 consecutively born children followed from birth, the cumulated incidence of asthma up to 11 years was 5.3% and the prevalence 3% between 10 and 11 years of age. The asthma début was prior to 1.5 years in 1/3 and before 3 years in 1/2 of the children. Among 59 children examined at 11.5-14.5 years of age, 16 (27%) had no prevalent asthma. On the other hand, inadequate medication was found in 13, undiagnosed chest deformity in four, and wheezing in seven children. All 54 tested children including those with no prevalent asthma had a PC20 histamine less than 8 mg/ml. Tread mill test gave a significant reaction in 15 children. Animal danders and pollens were the most common allergens giving reactions at skin prick test. Compared to one earlier Swedish study an increase in mite sensitivity was found. Cord-blood IgE concentration and a positive immediate family history of atopic disease had no predictive value for the severity of asthma at this follow-up. A new total asthma score including number of days with functional impact of asthma during the last year and present medication was used for group comparisons. Children with a high score at 11.5-14.5 years had more exercise-induced asthma, more concomitant allergic diseases, earlier asthma start, more chest deformity, hyperreactivity both on tread mill and histamine challenge tests, elevated IgE, positive Phadiatop and more reactions at skin prick test, especially to animals and mites. Bronchial asthma was found more often in children born during August through October, possibly due to unsuitable indoor climate and more virus infections during their first 6 months of life. The severity of the asthma was, however, not influenced by the month of birth. No significant differences were found between boys and girls regarding the age at début, asthma severity at follow-up, or bronchial histamine threshold levels.