Asthma mortality rates have declined only slowly since 1960, and recently have increased in some countries. Deaths in New Zealand's 5-34 year olds showed a sustained increase from 1977 to 1982. In the western United States, asthma mortality in older subjects increased from 1979 to 1982. In 15-44 year olds in England and Wales, mortality rates for females are now lower than in 1960, but rates for males are not. Current asthma mortality rates for Canadian 5-34 year olds are twice those reported a decade ago. A 2-year national study of all New Zealand patients under the age of 70 whose deaths were certified as due to, or related to, asthma found differences in verified asthma mortality rates among races (Maoris 18.9, Pacific Island Polynesians 9.4, and Europeans 3.4 per 100,000); the higher non-European rates explained 36% of the 'excess' New Zealand mortality compared with England. Errors in certification led to a 13% net overestimate of asthma mortality in both countries, but certification was highly accurate in 5-34 year olds. The increase in asthma mortality in the younger age groups is not an artefact of certification or coding, but is real and substantial. Although factors common to previous studies--poor patient compliance, inadequate assessment and therapy, overreliance on bronchodilators and underuse of corticosteroids, and delays in obtaining help--were frequently recognized, no single cause was found to explain the three-fold greater mortality in New Zealand compared with England.(ABSTRACT TRUNCATED AT 250 WORDS)