Trends in mortality in the age groups 1-5 and 6-11 months from 1966 to 1987 for Australia, Canada, England and Wales, New Zealand, and Sweden were examined. Mortality rates for ages 1-5 months differed appreciably between countries, with Sweden lower than all other countries examined. Rates have decreased in Australia, Canada, and England and Wales, but increased in New Zealand and Sweden. Mortality reported as due to the sudden death syndrome (SIDS) increased dramatically in all countries, although much of the increase was probably due to diagnostic transfer from respiratory diseases. Over 80% of SIDS deaths occurred in the age group 1-5 months and SIDS accounted for about half of all deaths in this age group. For developed countries total mortality in those aged 1-5 months was an indirect measure of SIDS mortality. A real increase in SIDS has thus occurred in Sweden and New Zealand and possibly in other countries as well. Mortality in the age group 6-11 months has approximately halved in all countries examined over the study period.