Introduction Monitoring of trends in asthma prevalence is essential to estimate the burden of disease and to assess the impact of asthma prevention programmes and changes in healthcare quality. The differences in secular trends in the prevalence of childhood asthma may relate to changes in diagnostic labelling, clinical practice and varying study methodologies.

Aim To compare secular changes in respiratory morbidity and asthma in children from lower socioeconomic areas of Merseyside between 1991 and 2006 using the same questionnaires and school populations.

Methods Four cross-sectional respiratory health surveys were completed in 1991 (n  =  1171), 1993 (n  =  2368), 1998 (n  =  1964) and 2006 (n  =  1074) among primary school children (5–11 years) attending the same 10 schools in Bootle and the same five schools in Wallasey, Merseyside.

Results The prevalence of doctor-diagnosed asthma increased from 17.7% in 1991 to 29.8% in 1998 and decreased to 19.3 in 2006. The prevalence of paternal and maternal asthma increased from 6.5% and 6.6%, respectively, in 1991 to 10.7% and 13.4% in 2006.

Conclusion There was an increase in the prevalence of childhood asthma and respiratory symptoms between 1991 and 1998 and a reversal of this trend from 1998 to 2006. It is unlikely that methodological differences between the surveys explain these changes as the same survey methodology and schools were used for this study at all time points. The secular variation in asthma prevalence requires continued community monitoring and surveillance in order to establish which factors are associated with these changing patterns.