The authors report the case of an 8-year-old girl who underwent a liver transplant at the age of 18 months because of biliary atresia. She was treated with cyclosporin for more than 5 years. Increased hirsutism prompted a change to tacrolimus therapy. During 11 months the mean tacrolimus level was 8.2 ng/mL. The patient was hospitalized because of an episode of Shigella infection and a threefold increase in tacrolimus level was measured. Despite a reduction of tacrolimus dose, the trough tacrolimus levels were in the range of 16.5 to 22.0 ng/mL during the subsequent 2 weeks. On resolution of the diarrhea, tacrolimus levels returned to those observed before the Shigella infection. It is suggested that the marked increase in tacrolimus levels observed in this patient is a direct result of the damage produced to the gastrointestinal mucosa by the Shigella infection.