The detection of LAV- or HTLV III-type viral particles in lymph node germinal centers from patients with the persistent lymphadenopathy syndrome (LAS) or the acquired immunodeficiency syndrome (AIDS)-related complex (ARC) is an important diagnostic factor in the prodromal stages of AIDS. These particles, the morphology of which is defined, are situated solely in the extracellular spaces delimited by cytoplasmic extensions of the dendritic reticular cells. Often few in number, they were found in 26 of the 30 lymph nodes studied, selected uniquely on the basis of light microscopic criteria (predominantly follicular lymphoid hyperplasia). The four negative nodes contained no, or fewer than two, germinal centers in the samples taken for ultrastructural study. The diagnosis of the LAS or the ARC was always confirmed clinically and biologically. Thus, lymph node biopsy and the corresponding ultrastructural study are important steps in the diagnosis of AIDS.