This is a review of the literature on the use of cycads as food and medicine, with special attention to their toxic properties. In the tropics and subtropics, where the plants are indigenous, their toxicity has long been known. Both gastrointestinal and neurological effects have been reported. Although several toxic components of the plants have been investigated, none has yet been shown to be responsible for specific effects. No lesion has been demonstrated to account for the progressive and apparently irreversible posterior paralysis which reputedly follows consumption of the plants by cattle. Current interest in the toxicity of the cycads has been stimulated by recognition of the high incidence of neurological diseases in an area of the world where they are used as food.