Several reports indicate that asthma mortality has increased during the last few decades. International comparisons reveal some striking differences in the pattern of asthma mortality. The authors investigated the asthma mortality rate in the Danish child and youth population 1973-1994 and studied the validity of death certificates. The authors reviewed all death certificates coded as asthma death in the International Classification of Diseases (ICD 8-ICD 10 (1994)) and adjacent respiratory code numbers for the age group 1-19 yrs. Hospital records and autopsy reports were assessed to validate the cause of death. Age-standardized and age-specific mortality rates were calculated. From 1973 to 1987 there was a significant upward trend in the mortality. On subdivision, this trend was limited to the age group 15-19 yrs. Generally the mortality rate decreased from 1988 to 1994. Four per cent coded as asthma were false positive. Twelve per cent were false negative asthma deaths, wrongly coded as due to other causes. Only 62% of all true positive death caused by asthma were appropriately coded. The number of false negative certifications increased with increasing autopsy frequency. Asthma mortality rates in Denmark increased in adolescents during 1973-1987 and decreased from 1988 to 1994. A possible explanation may be an increased awareness of asthma symptoms combined with a steadily improved treatment of asthma. Even in children and young adults under the age of 20 yrs, validity problems still make comparisons between countries difficult; even interpretation of national trends requires caution.