Nine cats experimentally infected with feline immunodeficiency virus (FIV) and six FIV-negative cats were necropsied to assess the effect of FIV infection on lymph nodes. The FIV infected cats were inoculated with 10(5) TCID50 21-22 weeks previously. The combined weights of all lymph nodes and the combined lymph node to organ weight ratios were significantly greater in FIV-infected cats when compared to uninfected cats. Additionally, by examining all nodes in the body, a regionally severe lymphadenopathy in FIV-infected cats was evident involving the lymph nodes of the hindlimb, forelimb, and head, in decreasing order of severity, with little evidence of enlargement in lymph nodes of the alimentary tract. Use of 99% confidence intervals showed that 9/9 FIV infected cats had enlarged lymph nodes of the hindlimb and forelimb region. In contrast, 7/9 and 3/9 FIV-infected cats exhibited enlargement of the nodes of the head region and alimentary tract, respectively. Similarly the combined weights of both left and right popliteal lymph nodes were enlarged in 9/9 FIV-infected cats whereas 0/6 in uninfected cats were not. The enlargement of the popliteal lymph nodes observed at necropsy was reflected microscopically by an increase in the size and number of germinal centres and an increase in the number of plasma cells, especially in the medullary cords. Because of the regional variation in lymph node size and numbers, it is suggested that the popliteal lymph node is a good indicator node for the assessment of lymph node status in FIV infection.