Feline immunodeficiency virus (FIV), discovered in 1987, shares a similar structure, genetic organization, lymphocyte tropism, and pathogenesis to the human immunodeficiency virus (HIV). During the acute phase of experimental FIV infection, mild lymphopenia and later lymphopenia and severe neutropenia have been described. The most frequent hematologic abnormalities include anemia, neutropenia, lymphopenia, and monocytosis. After infection, cats readily develop high titers of antibodies recognizing different FIV components. Antibodies can be detected by immunofluorescence assays, ELISAs, or immunochromatography tests. FIV infection can be diagnosed either directly, by detecting the virus, or indirectly, by demonstrating the presence of antibodies. Symptomatic treatment of infected cats should be directed at the clinical signs that arise from secondary infections.