BACKGROUND: Although there is convincing evidence that the prevalence of asthma among children has increased over the last three decades, it remains uncertain whether such an upward trend has occurred in adults. The aim of this study was to assess whether the prevalence of asthma has changed in young Belgian adults in recent years. METHODS: A retrospective analysis was conducted of available statistics from the Belgian Armed Forces for the period 1978-91 because conscripts who reported a history of past or current asthma at call-up examination underwent standardised assessment of non-specific airway responsiveness by military chest physicians. Exemption from military service due to asthma was strictly based on the objective evidence of airway hyperresponsiveness. RESULTS: A mean of 48,331 conscripts aged 17-31 years were examined annually from 1978 to 1991. The prevalence of reported asthma rose from 2.4% in 1978 to 7.2% in 1991, while the proportion of asthmatics with airway hyperresponsiveness remained little changed at 48.4% in 1978 and 51.4% in 1991. CONCLUSIONS: The observed increase in prevalence of reported asthma was not accompanied by a decrease in the proportion of conscripts with objectively measured airway hyperresponsiveness. These observations provide supporting evidence that the increase in asthma symptoms was not simply due to reporting bias.