The first case of immunoblastic lymphadenopathy developing in a child which progressed to immunoblastic sarcoma is reported. The sarcoma cells showed light and electron microscopic features of transformed lymphocytes (immunoblasts) but it was not possible to establish their B-cell origin using a peroxidase-antiperoxidase technique for the demonstration of intracellular immunoglobulins. In the affected lymph nodes there was marked proliferation of reticulum and endothelial cells both of which contained numerous intranuclear inclusions which may be of viral origin. Ultrastructural studies suggest that the amorphous eosinophilic interstitial material, an important diagnostic morphological feature of immunoblastic lymphadenopahy, results from the oblique sectioning of the elongated and branching cytoplasmic processes of reticulum cells. It is postulated that in immunoblastic lymphadenopathy the proliferation of reticulum and endothelial cells may be the primary event, perhaps stimulated by viral infection and that the intense lymphocytic and plasmacytic infiltration and sarcomatous transformation occur as secondary phenomena.