AbstractScientific evidence has accumulated to show that chronic atrophic gastritis (CAG) is a precursor of gastric carcinoma, especially its intestinal histologic type; thus the etiology of CAG is of interest. Data on 110 families (557 individuals) collected as part of a large cohort from the Narino region of Colombia, South America, are analyzed to determine the familiality of CAG as a risk factor, and the possible involvement of a major gene in its etiology. We found that age and having an affected mother are important risk factors. In the sample, 45 are affected; 56 of individuals above 30 are affected, whereas only 28 of those 30 and under are affected; 48 of those with affected mothers are affected, but only 7 of those with unaffected mothers are affected. A positive spouse association was confounded with age. Sex and an affected father are not significant risk factors. The genetic (segregation) analysis showed Mendelian transmission of a recessive autosomal gene with penetrance dependent on age and mother's CAG status. Homozygous recessives account for an estimated 61 of the sampled population and have penetrance reaching 72 at age 30 if the mother is affected, and 41 if the mother is not affected. Carriers and non‐carriers, who make up an estimated 39 of the sampled population, have an appreciable estimated risk after age 50. The environment, particularly diet, as the sole determinant of CAG needs re‐evaluation; some combined action of genes and environment seems more plausi
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