Ultrastructural study of rectal mucosa was performed in 6 patients with AIDS related complex (ARC) and in 10 patients with AIDS. There were 16 men (mean age: 39.1 years): 8 homosexuals, 3 Haitians, 2 Africans and 3 IV drug abusers, all having significant titers of LAV antibodies. Two types of ultrastructural markers were observed: tubuloreticular structures (TRS) were found in endothelial cells, lymphocytes and macrophages in 1/6 ARC patients and 10/10 AIDS patients. TRS have already been described in various pathological situations and seem to be related to alpha-interferon, test tube and ring shaped forms (TRF) were observed in lymphocytes and macrophages in 0/6 ARC patients and 6/10 AIDS patients. TRF have been rarely reported previously. The 6 AIDS patients with TRF were 4 homosexuals and 2 Haitians. In two of three of these patients in whom repeated ultrastructural studies were performed, the same markers were found. All AIDS patients with TRF died within a mean time of ten months after the diagnosis had been established, whereas 1/4 AIDS patients without TRF died within seven months and the three others survived for more than 14 months. These data suggest that the association of TRS and TRF in the rectal mucosa could be specific of AIDS. In LAV/HTLV III retro-virus infection, this association is an aggravating factor.