A 4-year continuous screening for hepatitis B antigen (HBAg) in all patients admitted to a Copenhagen general hospital revealed 253 antigen-positive cases among 99000 patients (2.5%). 97 of the patients had acute or chronic hepatitis, while there were no signs of liver disease in 156. The "healthy" carriers showed a wavy age distribution, indicating an increased transmission of HBAg in age groups born 1920-30 and 1940-60. Hepatitis B antibody (HBAb), measured by a radioimmunoassay, occurred in 8.6% of 1000 patients. HBAb also showed an uneven age distribution with a peak prevalence of 17.5% in the age group born 1910-30. These figures indicate that the prevalence of HBAg and hence presumably also the hepatitis B virus has fluctuated within this homogeneous population, probably related to changes in the sociohygienic conditions. Determination of HBAg subtypes D and Y in 161 patients indicated that the Y-subtype is now being introduced into a formerly subtype D area by means of drug addiction, tourism or workers of East-Mediterranean origin.