Seeing the same transmission pattern of HIV and HBV coinfection by these two agents is not an uncommon feature. Immunity impairment due to HIV infection can be the cause of a higher rate of HBV replication with less intensive liver damage and less effective immune response to HBV, while the pathological course in both infections involves elevated levels of circulating immune complexes (CIC). These were the reasons for us to examine the frequency of HBsAg involvement as the antigen component of circulating immune complexes formed in sera of HIV-infected patients in different stages of HIV disease. We tested 67 sera of HIV-positive patients in different stages of HIV disease for the presence of HBsAg and HIV antigen p24 (with and without acid dissociation of immune complexes), for the presence of anti-Hbc antibodies and circulating immune complexes. HBsAg was positive in 13.8% sera prior to and 33.8% after acid pretreatment. Anti-HBc antibodies were present in 76.9% serum samples tested. Fifty percent of sera were positive for both HBsAg and p24 antigen after dissociation of immune complexes. The level of CIC was elevated in 65.9% of sera. Our results suggest that HBsAg is commonly associated in immune complexes formed in the sera of HIV-infected patients and that they may simultaneously contain HIV and HBsAg in patients coinfected with both agents. This may contribute to their mutual interaction and influence the diagnosis and follow-up of patients.