The incidence of HBsAg in several areas of Romania was studied by 253,460 routine determinations performed from 1973 to 1975. HBsAg incidence was of 1.5% in blood donors and reached the highest value in acute viral hepatitis patients undifferentiated according to the clinical form. The values obtained were in agreement with the results of other studies covering more limited zones of the country. Considerable differences in HBsAg incidence depending on geographic area were observed.