The gastric acidity of young to elderly Japanese subjects from 1989 to 1999 was assessed and compared with that obtained in 1984, using GA-Test capsules containing acid-dissolving granules of riboflavin. The percentage of achlorhydric subjects increased with age as observed before, however, an over all decrease in all age categories year by year was noted. The percentage of achlorhydric subjects aged 50 years in 1995-1999 was about 40%, which was lower than that (60%) in 1984. However, such a chronological change was not observed when the percentage of achlorhydric subjects was determined according to birth year, indicating that it is related to the birth year of subjects. The percentage of achlorhydric subjects correlated with infection by Helicobacter pylori. Considering the high percentage of achlorhydric elderly, bioavailability and bioequivalence studies should be performed taking into consideration the effects of gastric acidity on the in vivo performance of drug products.