The Iberian lynx (Lynx pardinus) is considered the most endangered felid species in the world. To date, less than 200 animals remain in the wild. Low numbers and genetic uniformity may contribute to render this species particularly susceptible to infectious diseases. Different pathogens have been identified in Iberian lynxes; including several feline bacterial and viral agents. Within a 6-month period starting in December 2006, 12 lynxes living in the northern part of the Doñana area were found to be infected with feline leukemia virus (FeLV). Eleven of these animals were antigenemic, and four of them died in the wild in less than 6 months since the first infected animal had been discovered. The remaining viremic lynxes were captured and allocated to a quarantine center to stop the spread of the infection. An additional three animals died shortly in the quarantine center due to acute anemic disease. Sequencing of the envelope surface unit gene revealed a common origin for the FeLV found in all lynxes. The sequences were closely related to FeLV-A/61E, originally isolated from cats in the USA. Our data demonstrate that, similarly to FeLV, the introduction of a new or particularly pathogenic infection brought into the small population of Iberian lynxes by other wild carnivores or feral cats and dogs roaming in the same habitats have severe consequences. It could result in epidemics that have the potential to eradicate the entire lynx population.