Celiac disease is a genetically determined intolerance to gluten that results in villous atrophy in the small intestine. Because celiac disease occurs in families, relatives of affected individuals are tested for the disease. However, there are no evidence-based guidelines for when, or how often, to test relatives. Our goal was to determine if one-time screening of relatives is sufficient. Of 171 family members with an initially negative endomysial antibody who were tested on more than one occasion, 6 (3.5%) were positive on repeat testing. The average time to seroconversion was 1.7+/-1.2 years (range, 6 months-3 years 2 months). Only one of the seroconverters had diarrhea; the remainder were asymptomatic. None of the patients had a change in symptoms between testing. We conclude that one-time testing for celiac disease among families with affected members is insufficient. Repeat testing should occur irrespective of the presence of symptoms.