BACKGROUND: Deficiency of vitamin B12 raises the serum and tissue levels of homocysteine. Atrophic corpus gastritis results in impaired secretion of intrinsic factor and may lead to malabsorption of vitamin B12 in the intestine. We examined how common an undiagnosed vitamin B12 deficiency is among elderly men in the general population and, in particular, how often this deficiency is related to atrophic corpus gastritis. METHODS: The serum level of pepsinogen I (S-PGI) was assayed in a population-based sample of 12,252 men (age 51-65 years) from two cities in Finland. In this sample, all 635 men with S-PGI < 25 microg/l formed Series A ('males with atrophic corpus gastritis'). Series C (controls--'males without atrophic corpus gastritis)' with a non-atrophic gastric corpus was formed as a random sample of men (n = 402) with S-PGI > or = 50 microg/l. Serum levels of vitamin B12 (S-B12), folate (S-Fol), total homocysteine (S-Hcy) and Helicobacter pylori antibodies (S-HpAb) were assayed in all, or in large subsamples, of the men in Series A and C. RESULTS The men in Series A had significantly lower S-B12 and S-Fol levels than those in Series C. In Series A, 172 of 613 men tested (28%) had S-B 12 < 170 pmol/ 1, and 133 men (22%) had S-B 12 in the range 170-219 pmol/l. The corresponding prevalences were 7% (P < 0.001) and 17% (P < 0.001) in Series C, respectively. The mean S-Hcy was significantly higher in Series A in men with low S-B12 than the mean S-Hcy in Series C in men with normal S-B12. The prevalence of S-Hcy > 15 ,micromol/l was 27% in Series A and 15% in Series C (P < 0.05; chi2 = 4.63). Among subjects with S-B 12 < 220 pmol/l, 46% (104 of 226 men tested) in Series A and 16% (16 of 99) in Series C had S-Hcy > or = 15 micromol/l (P < 0.001). The mean S-Hcy was significantly (P < 0.001) higher in men with S-B12 in the range 170-219 pmol/l in Series A (mean 14.6 +/- 5.0 micromol/l) than in Series C (11.3 +/- 3.0 micromol/l). It was extrapolated that 2.5% of men in the age group 51-65 years in the present study population had a low S-B12 (< 220 pmol/l) level that associated with atrophic corpus gastritis. Of these men, 72% (128 of 179 tested) had an elevated S-HpAb level. CONCLUSIONS: Low S-B12 related to atrophic corpus gastritis is relatively common (prevalence 2.5%) among elderly males in the general population. An ongoing H. pylori infection occurs in three-fourths of these cases.