In January and February 1991, the health officer in the parish of St. Ann, Jamaica, received reports of eight persons with toxic hypoglycemic syndrome (THS), an illness associated with consumption of unripe ackee fruit and, possibly, renta yam; two cases were fatal. On July 25, the Jamaican Ministry of Health (JMH) contacted CDC for assistance in investigating the continued occurrence of THS; the collaborative JMH and CDC epidemiologic investigation focused on characterizing the epidemiology of THS in Jamaica and assessing the role of ackee fruit, renta yams, and other factors.