Retinoids are a class of compounds structurally related to vitamin A. More than 2500 retinoids have been synthesized and tested for biological activity. Nine of these were evaluated as agents for cancer chemoprevention by an international group of experts at a meeting held in March 1999. Some are agents in clinical use for psoriasis or severe acne, and others are undergoing clinical trials for prevention of second primary tumours in patients with head and neck tumours or breast cancer, while all-transretinoic acid is used in the treatment of acute promyelocytic leukaemias. The toxicity of these agents was subjected to close scrutiny by the working group, which concluded that, for nearly all these agents, even if they are eventually shown to be effective for cancer prevention in humans, their toxicity is likely to limit their use.