The clinical, functional, and histopathological effects of 18 g additional gluten intake daily for 4 wk was studied in 13 healthy siblings of Spanish children with celiac disease, including two pairs of discordant monozygotic twins. Five of the 13 children were HLA-DR identical to the celiac sibling, 5 shared only one HLA-DR antigen with the celiac sibling, and 3 had completely different HLA-DR antigens than their respective celiac brothers or sisters. Clinical evaluation and functional tests (routine blood, xylose absorption, and fecal fat excretion studies) were performed before, during, and after gluten challenge. A jejunal biopsy specimen was taken at the end of the 4-wk period of gluten supplementation. No clinical abnormalities were found during the period of the study and there was no significant decrease of xylose absorption. Fecal fat excretion studies gave normal results, both before and after gluten challenge. The high gluten diet did not lead to histopathologic abnormalities in any of the jejunal biopsy specimens, which showed a normal range of crypt to villus ratio and surface-cell height. The present results do not support the view that excessive gluten intake is toxic for individuals who are genetically predisposed but do not have overt celiac disease. The findings also suggest that other factors besides HLA-DR antigens and gluten intake are important for expression of the disease.