Feline leukemia has been the subject of intense research over the last decade following its recognition as an infectious disease, and its potential significance as a model for the study of human disease. The evidence in favor of a causal relationship between a virus and the disease is in the epidemiologic data obtained in feline leukemia virus (FeLV)-positive leukemia cluster households. When FeLV infection results in neoplastic disease, classification is based on the cell type that has undergone malignant transformation. Domestic cats are exposed to FeLV following prolonged contact with the saliva or urine of naturally viremic cats. Control of the FeLV dose and strain, coupled with variation of the host age at time of FeLV inoculation, will result in populations of cats destined to develop either viremic or immune FeLV infections. FeLV is a unique retrovirus in that it is a horizontally transmitted infectious agent which infects an outbred population of cats.