To assess the clinical implication of concomitant HBsAg and anti-HBs in patients with HBV infection, we studied the longitudinal behavior of HBsAg, anti-HBs and their subtypes and the correlation of clinical events in 25 patients with concomitant HBsAg and anti-HBs. The results were as follows: 1) Concurrent markers were persistent in 12 (57.1%) out of 21 patients with chronic liver disease. In two patients, anti-HRs was detected intermittently during the follow-up period. In each, the recurrent antibody was persistently heterotypic, and the appearance, disappearance and reappearance of anti-HBs were not associated with alterations in the clinical course. 2) The mean level of anti-HBs was 16.6±17.7 SN ratrio and 61.9% of the patients were less than 10 SN ratio. 3) In the majority of patients, HBsAg subtype ad was accompanied by anti-y. In addition, in two patients with intermittent concomitance the recurrent amtibody was persistently heterotypic. 4) During the follow-up period, acute exacerbations of the disease occurred in four out of 21 patients with chronic hepatitis. All patients were initially HBsAg subtype ad and remained so during and after exacerbation. Therefore, the possibility of reinfection with HBV of a different subtype was not supported. In conclusion, concomitant HBsAg and anti-HBs are not uncommon in patients with acute and chronic hepatitis B and do not reflect changes in the clinical courses. The concomitance is of no practical or prognostic significance, and therefore, the attention devoted to it may be clinically unnecessary.