Routine mortality statistics show that coronary heart disease (CHD) death rates have declined consistently in Auckland men since 1968; in women, death rates declined between 1968 and 1986 but since 1981 there may have been a reduction in the rate of decline. Data from CHD registers conducted in Auckland, New Zealand in 1974, 1981, and since 1983 as part of the WHO MONICA Project, have been used to investigate the validity and reasons for the decline in the age group 35-64 years. In Auckland age-standardized sudden coronary death rates in men declined by approximately 2% per year between 1974 and 1986; there was no apparent decline in women. There was also an indication of a decline in age-standardized definite myocardial infarction rates but again only in men; 28 day case fatality in patients with a definite myocardial infarction has not changed significantly in the period 1981-1986. These results validate the mortality trends based on death certificates and in particular the differing recent trends in men and women. The decline in CHD mortality in men without a concomitant change in case fatality and the lack of recent decline in women, suggest that changes in the natural history of the disease rather than treatment are responsible for the mortality trends. Since disease events are rare in absolute numbers, long-term monitoring of coronary heart disease in large population groups will be necessary to usefully study disease trends, particularly in women.