Thermal burns as well as hyperbaric oxygen (HBO) may cause immuno-suppression. This is one of the reasons why there is some controversy in the literature regarding adjuvant HBO treatment for thermal burn patient, despite the fact that HBO is known to decrease edema formation and possibly inhibits the progression from second to third degree burns. In this study, lymphocyte subpopulations were labelled with monoclonal antibodies W3/25 for helper cells, and OX-8 for cytotoxic/suppressor cells, to determine changes following early burn wound excision and acute or chronic HBO treatment in a 10% full-thickness burn model in rats. Lymphocyte subpopulations were extracted from blood and spleen on day 1, 8, and 15 following burn and/or treatment. W3/25 cells did not show any significant changes in blood or spleen over time. Significantly lower OX-8 cell counts were found in the group with burn + excision + chronic HBO treatment on day 8 and 15. Acute or chronic HBO treatment alone did not produce evidence of immuno-suppression.