Elevated levels of serum gastrin as a consequence of gastric achlorhydria are characteristic of clinical pernicious anemia. In a largely male hospitalized population with an average age of 64 years, 7.7% had low levels of serum B12 (less than 170 pg/mL); only 2.5% of these had frank pernicious anemia. In an attempt to separate a subgroup with low levels of serum B12 in whom pernicious anemia may later develop, their serum gastrin levels were determined. Twenty-two percent had high values and, of these, 70% had low B12 absorptions. In patients with low serum B12 levels, serum gastrin assays may be useful in determining those in whom clinical pernicious anemia seems likely to develop.