The Iberian lynx (Lynx pardinus) is considered the most endangered felid species in the world, less than 250 animals left. The narrow genetic basis may contribute to render this species particularly susceptible to infectious diseases. Within a six-month period starting in December 2006, a feline leukemia virus (FeLV) outbreak of surprising virulence killed about 60% of infected animals. Sequencing of the FeLV envelope surface unit gene revealed a common origin in all lynxes. The sequences were closely related to FeLV-A/61E, originally isolated from a cat in Colorado and only mildly pathogenic in domestic cats. Nevertheless, a particular virulence of the FeLV strain found in Iberian lynxes could not be ruled out. In order to evaluate the virulence of the lynx FeLV, we assessed the disease-inducing potential of the Iberian lynx`s FeLV strain in its probable original host, the domestic cat. Intraperitoneal inoculation of specified pathogen-free (SPF) domestic cats with FeLV-infected Iberian lynx blood did not lead to a particularly severe outcome. Thus, the FeLV epidemic in the Iberian lynxes seems to be more related to a potential primary immunodeficiency of these animals than to an extremely virulent FeLV strain.