Herbal medicines are used in health care around the world and may increase in importance. There is much uncertainty, however, with regard to their composition, efficacy and safety. There is substantial evidence that herbal medicines can cause serious adverse reactions, but more data are needed as regard their nature, frequency and preventability. In this respect the Uppsala Monitoring Centre of the World Health Organization can play a crucial role. Better reporting of adverse reactions to herbal medicines is needed, in particular with regard to the precise identity and composition of these products. A consistent use by producers, regulators and reporters of the international Latin binomial nomenclature and the use of the new Herbal Anatomical Therapeutic Chemical (ATC) classification are recommended. Copyright (C) 2000 John Wiley & Sons, Ltd.