Observing a lack of response to orally administered drugs in a patient with Behçet's disease, we studied the absorption of amitriptyline, diazepam, carbamazepine, phenytoin, and acetaminophen in this patient after single and (or) multiple dose administrations. The relative oral/intramuscular bioavailability of amitriptyline was only 13%, and the steady-state concentrations of this drug on four consecutive days were acutely subtherapeutic (i.e., 3.6, 3.7, 3.9, and 3.7 micrograms/L). The concentrations of diazepam, phenytoin, and acetaminophen in plasma were nonmeasurable. Examination of the gastrointestinal tract by endoscopy and by light and electronic microscopy of a biopsy section revealed inflammatory and vascular changes in the duodenum. In the absence of clinical evidence for malabsorption syndrome, we believe that the decreased drug absorption observed in this patient was caused by inflammatory changes associated with Behçet's syndrome.