The relationships between the medication prescribed and four measures of morbidity were assessed in 150 Dutch patients with asthma in general practice. Undertreatment was operationalized as a discrepancy between medication and morbidity severity. Of the patients, 11.3% were undertreated as measured by the number of days during which activities were interrupted by asthmatic symptoms. Additional measures such as the number of nights interrupted by asthma, number of attacks and episodes of wheezing, indicated that 28.0%, 24.0% and 34.5% of the patients respectively were undertreated. It is concluded that asthma seems to be undertreated in general practice in The Netherlands, a finding which is consistent with research on this subject in the UK and other countries. Possible reasons for the persistence of undertreatment of asthma are discussed, and suggestions for overcoming it are offered.