Thermal injury is associated with dysfunction of host defense systems. The present study used flow cytometric immunofluorescence analyses to investigate changes in number and phenotype of lymphocytes in seven different lymphoid compartments at 2, 6, 12, 24, 48, and 60 days after 50% total body-surface area thermal injury in the rat. Relative to sham-injured control rats, at postburn day 2, significant lymphopenia was observed in the peripheral blood along with depletion of lymphocytes from the spleen and thymus. By day 6 after injury, lymphocytes in the bone marrow and cervical lymph nodes decreased significantly while numbers in the spleen and thymus remained depressed. Splenic and cervical node lymphocyte numbers normalized by day 12, the bone marrow and thymus numbers still were significantly lower than control, and a 6.5-fold increase in number of lymphocytes was observed in the nodes draining the burn wound, pooled axillary, brachial, inguinal, and lumbar lymph nodes. At day 24 after injury, the thymus and bone marrow virtually were depleted of lymphocytes, the mesenteric lymph nodes manifested a significant decrease, and lymphocytes in the nodes draining the burn wound continued to increase in number. This same pattern was maintained on day 48, but numbers of lymphocytes in the mesenteric nodes normalized. At day 60 after injury, lymphocyte numbers in all tissues were normalized, but the spleen and nodes draining the burn wound where increased numbers compared to control persisted. Cell-surface phenotyping was performed on all lymphoid tissues at all time intervals to determine the percentages of lymphocytes comprising the following subsets: Ia+ cells (B cells and activated T cells), T cells, T-Helper/Inducer cells (T-H/I), and T-Suppressor/Cytotoxic (T-S/C) cells. Although changes in lymphocyte subset percentages were complex, they could be divided grossly into two phases. First, all compartments showed significant phenotypic changes in the first six days after burn. With the exception of the nodes draining the burn wound and the blood, this was followed by a return towards normal on day 12. The second phase then ensued with significant phenotypic changes again occurring in most tissues from days 24 to 60 after injury. These studies demonstrate that burn injury results in dramatic alterations in lymphocyte numbers and subset percentages in different lymphoid compartments. Immune alterations observed following thermal injury may be due, in part, to a redistribution of the cellular elements responsible for generation of the immune response.