A total of 100 post-mortems were done on patients clinically diagnosed as dengue haemorrhagic fever from Rangoon Children's Hospital. Histopathological changes in bone marrow, thymus, spleen, lymph nodes and other associated tissues of the immune system were analysed and correlated with the clinical picture and serology results. The major changes in cases with a positive serology result for secondary dengue infection consist of hypoplasia of the bone marrow, acute atrophy and wasting of the thymus, atrophy and depletion of cells in the periarterial lymphatic sheaths of the spleen and the paracortical areas of the lymph nodes. The tissues affected are the thymus-dependent areas of the spleen and lymph nodes, and the thymus itself. Thymus-independent areas of the secondary lymphatic tissues are also affected but to a lesser extent. The pathological observations suggest that immunodepression may be an integral part of the pathophysiology of dengue haemorrhagic fever.