Feline leukemia virus (FeLV) infection is common among cats where contact is high. The virus can be transmitted readily between cats. It causes a variety of haemopoietic and lymphoid neoplasms; the most common types are alimentary, multicentric and thymic lymphosarcoma and lymphatic leukaemia. The virus is involved in the aetiology of certain other diseases including anaemia, glomerulonephritis and an immunosuppressive syndrome which predisposes cats to intercurrent infections. Many infected cats mount an immune response and do not suffer from any of these. The immune status is shown by serum antibody levels to feline leukaemia virus associated cell membrane antigens. Cats with a titre of 32 or more are most unlikely to suffer any ill effects and may eliminate the virus infection. The outcome of infection in an individual cat depends on the immunological competence of the cat, the dose of virus received and its ability to induce immunosuppression. FeLV infection can be detected by examination of tissues by electron microscopy, and by culture of virus from plasma and other tissues. In the United States, a method is now in use for the detection of leukaemia virus antigen in peripheral blood leukocytes; this is carried out on ordinary blood films. Successful prototype vaccines have been developed against FeLV. This paper describes the natural history of the virus, the diseases in which it is implicated and discusses recently developed diagnostic methods.