In a healthy population pepsinogen A and pepsinogen C increase with advancing age. As pepsinogen A and C are raised in chronic superficial gastritis which is caused by H pylori infection, we investigated whether H pylori is responsible for the age related increase of pepsinogen A and C. In H pylori positive blood transfusion donors serum pepsinogen A (mean (SD) 73 (35) micrograms/ml v 52 (19) micrograms/ml, p much less than 0.01) and C (mean (SD) 24 (13) micrograms/ml v 10 (7) micrograms/ml, p much less than 0.01) concentrations were significantly higher than in H pylori negative blood transfusion donors, while the serum pepsinogen A:C ratio mean (SD) 3.5 (1.4) v 6.2 (3.4), p much less than 0.01) was significantly decreased because of a relative greater increase in serum pepsinogen C in H pylori positive blood transfusion donors. Analysis of variance showed that pepsinogen A and C concentrations differed significantly in the different age groups (p much less than 0.01) when we considered all blood transfusion donors and H pylori positive blood transfusion donors, the mean pepsinogen levels being highest in the older age categories. In H pylori negative blood transfusion donors no such age related difference in pepsinogen A and C could be shown. In H pylori positive blood transfusion donors a weak positive but significant correlation between pepsinogen A and C and age could be shown (r = 0.30; p = 0.01 and r = 0.31; p = 0.01 respectively). In H pylori negative blood transfusion donors no correlation between serum pepsinogens and age was found. We conclude that the age related increase in serum pepsinogen A and C described in healthy control populations is caused by an increasing prevalence of H pylori infection. Serum pepsinogen A and C concentrations in patients should therefore be related to the presence or absence of H pylori infection.