Changes in lung function due to childhood asthma have been reported to occur before school age, and to persist throughout life. The aim was to assess the relationship between aspects of lung function and asthma over time in 4,089 children participating in the large population-based birth cohort BAMSE. Questionnaires were administered at 1, 2, 4, and 8 years of age. At 4 and 8 years, children were invited to a clinical examination, in which 2,965 and 2,630 children participated, respectively. The examinations included blood sampling for evaluation of sensitization to airway allergens (n = 2,053), peak expiratory flow (PEF) measurements at 4 and 8 years (n = 1,957), and forced expiratory flows (n = 2,455) at 8 years. Asthma onset before the age of 4 years, but no thereafter, was at 8 years associated with impaired spirometric flows. This was seen irrespective of symptom presence after the age of 4. Reduced PEF growth between the age of 4 and 8 was seen only for the group of children with early onset transient asthma, while an association between sensitization and lung function was only seen in the late-onset asthma group. In conclusion, school children with asthma have reductions of spirometric flows when categorized as persistent or transient early onset asthma, even if this latter group of children is completely symptom-free at school age.