Many studies of asthma mortality rely on official registration. The aim of this study was to evaluate the accuracy of death certificates, where asthma was coded as cause of death. In a 12-month period, medical information on all subjects with asthma officially coded as the underlying cause of death in Denmark, was obtained by reviewing hospital records, contacting general practitioners and sometimes close relatives. A panel of four pulmonologists each examined the obtained information and independently assessed the cause of death. Of a total of 218 death certificates, 39 were excluded as the cause of death could not be validated. In 16 (9%) of the subjects death from asthma was judged to be the definite cause of death and in 12 (7%) death from asthma was possible. Of 151 non-asthma deaths coded as due to asthma, 109 were judged to have suffered or died from COPD and 14 from heart disease. The accuracy of Danish death certification in asthma deaths is poor, especially in the elderly, where COPD is often classified as asthma. We conclude that the true asthma mortality in Denmark is substantially lower than officially recorded.