Increased intestinal permeability to several specific molecular probes has been observed in patients with Crohn's disease and their first-degree relatives. A positive family history is also a potent risk factor for inflammatory bowel disease. Although it has been argued that increased permeability in relatives may confer an increased future risk of developing Crohn's disease, long-term follow-up of such family members has been lacking. We describe a 24-year-old woman with a positive family history of Crohn's disease who had an elevated gut permeability to (51)Cr-EDTA at age 13, as part of a cross-sectional cohort study in patients and their first-degree relatives. She was asymptomatic at the time, and extensive investigation found no evidence of microscopic or macroscopic Crohn's disease. Repeat investigation because of symptom onset at age 21 revealed ileocolonic Crohn's disease, which required treatment with systemic corticosteroids to induce a clinical remission. In this case, a permeability defect was clearly identified to precede the onset of Crohn's disease in a subject at increased risk. This observation provides support for the hypothesis that increased gut permeability to macromolecules is an early step in the pathogenesis of this disorder.