HLA-A B and DR typing were performed in 77 patients with AIDS related complex (ARC)--69 lymphadenopathy associated syndrome and 8 thrombocytopenic purpura LAV/HTLV III related--and 21 symptom free homosexual males. A significant increase in the frequency of HLA DR5 antigen was observed in patients with ARC mainly in purpura thrombocytopenic patients. We suggest that increase of HLA DR5 antigen support the view that DR5 antigen could be one of the factors necessary at the spreading out clinical symptoms.