Of 228 consecutive hepatitis B surface antigen (HBsAg)-positive patients who had simultaneous hepatitis B surface antibody (anti-HBs) determinations, HBsAg and anti-HBs were found concurrently in 73 (32%). Concurrence was found with greater frequency in patients with chronic active hepatitis than in those with acute hepatitis (36/57 vs 13/38, p less than 0.02). Patients with chronic active hepatitis had concurrent markers more commonly than those with chronic persistent hepatitis or asymptomatic carrier state (36/57 vs. 24/133, p less than 0.001). No major risk factors were identified. Hepatitis B e antigen was detected more frequently in patients with concurrence (68% vs. 42%, p less than 0.01). Subtyping was possible in 30 patients with chronic infection, including 18 with chronic active hepatitis, 7 with chronic persistent hepatitis, and 5 with a carrier state. In 25 patients, HBsAg subtype ad was found with antibody to subdeterminant y and in four instances, HBsAg subtype ay was found with antibody to subdeterminant d. Only 1 patient had homotypic HBsAg and anti-HBs. Of 38 patients who had successive determinations, concurrence was constant in 29. In 9 others, anti-HBs was detected intermittently and the heterotypia of the recurrent antibody remained constant. Antibody to hepatitis delta-virus was not detected. We conclude that concurrent HBsAg and anti-HBs are found frequently in acute and chronic hepatitis B. The markers are commonly heterotypic in chronic disease. The presence of heterotypic markers is not associated with specific risk factors or delta-infection. Concurrence is associated with evidence of viral replication and features of active inflammation.