Seven children ages 1 1/2 to 12 years with congenital pernicious anemia were detected in an extended Mexican family. All affected children had megaloblastic anemia accompanied by low serum B12 and normal serum folate levels. Gastric fluid analysis in six patients revealed normal gastric acidity and absent intrinsic factor. Serum antibodies to intrinsic factor or parietal cells were also absent. Schilling tests performed in six of the seven patients yielded abnormal results. Of the three patients in whom gastric biopsy was done, two had normal histologic findings (including examination by electron microscopy) and one had mild atrophy. All patients responded rapidly to parenterally administered vitamin B12 therapy. In addition, 170 family members were screened for the defect with complete blood counts and serum B12 levels. Such screening detected pernicious anemia in two of the children, but no other abnormalities that could be attributed to pernicious anemia were found in other family members. Based on the family pedigree, autosomal recessive inheritance is likely. The variability of age of presentation in this family is noteworthy and suggests that expression may be modified by still undefined factors.