Rates of stomach cancer vary from place to place within England and Wales. To determine whether this reflects influences acting earlier or later in life, we have analysed mortality from the disease by county of birth and county of death. Among 749,035 men and women who died during 1969-72 in a different county from that in which they were born, proportional mortality from stomach cancer was more closely related to county of birth than of death. This association with place of birth was found in migrants both out of and into high-risk areas. We conclude that studies seeking to explain local differences in the incidence of stomach cancer within England and Wales should focus on the environment of patients in their youth.