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Study objective-To describe time trends in gastric cancer incidence in Zaragoza and Navarre, and to investigate time period and birth cohort as determinants of such trends. Design-Cases from two registries were grouped into five year intervals and the following were calculated: age specific and sex specific incidence rates, age adjusted incidence rates, cumulative rates, and the male to female ratio. The decline in gastric cancer incidence and mortality is a worldwide phenomenon coinciding with the second half of this century."2 Nevertheless, it is still one of the most frequent cancers in the world and occurs twice as frequently in men than in women.3Gastric cancer exhibits noticeable international differences. European population based registries with the highest and lowest gastric cancer incidence rates for both sexes are St Petersburg, Russia (52.8 cases per 100 000 person-years for men and 25.3 for women) and Tam, France (8.3 for men and 3.4 for women), respectively.4 In Spain, gastric cancer not only registers high levels of incidence and mortality (above the European Community average4 5) but in 1985 was the second leading cause of cancer deaths in men and the third in women.6 As yet, no complete explanation has been forthcoming as to the important geographical differences that exist between and within countries or the factors associated with the development of this tumour.

The substantial cancer increment observed points out the inefficacy of primary prevention policies, the importance of studying cancer incidence for long time periods and the need to increase the population coverage of Spanish cancer registries.

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