To evaluate possible sex differences in prevalence, diagnosis and treatment of asthma, and influence on lung function associated with asthma severity in a population-based birth cohort (BAMSE) of 4089 children. At 4-y follow-up, 92% responded to a questionnaire on symptoms of asthma, current medication and doctor's diagnosis of asthma. A total of 2965 children participated in clinical testing, including measurements of peak expiratory flow (PEF), and were assigned to groups of asthma or no asthma by reported airway symptoms. Children in asthma groups had lower PEF readings compared to healthy children. This effect was most pronounced for both sexes in the persistent group (boys p<0.05, girls p<0.001) and for girls in the transient group (p<0.01). A doctor's diagnosis of asthma did not significantly differ between boys and girls, but significantly more boys than girls had inhaled corticosteroids, even when stratifying by frequency of symptoms. These results suggest that when 4-y-old children are stratified according to common diagnostic criteria, girls have a larger effect on PEF of asthma symptoms and are less frequently treated compared to boys.