Immunosuppressive therapy in patients after liver transplantation requires careful monitoring of blood levels for immunosuppressive agents such as cyclosporin A. A variety of drugs are capable of interfering with the metabolism of cyclosporin A. We observed a 63-year-old patient who received a liver allograft for cryptogenic liver cirrhosis in 1998. This patient developed severe acute rejection 14 months after transplantation which was associated with a sudden drop in cyclosporin A levels. Two weeks previously, he had started taking the herbal drug Hypericum perforatum (2 x 900 mg/day) for increasing episodes of depression. The cyclosporin A dosage later had to be doubled, which caused some side effects. Finally, an assessment of oral cyclosporin A resorption suggested an enhanced cyclosporin A metabolism. Hypericum perforatum was stopped. Both cyclosporin A dosage and blood levels immediately returned to normal. The liver function recovered completely. In conclusion, this observation is a previously undescribed drug interaction of a widely used herbal drug (Hypericum perforatum, i.e. St. John's wort) in a patient after liver transplantation.