A prospective study of the natural history of acute hepatitis B was performed in 38 patients. Fatigue started median 4 weeks, abdominal symptoms median 3 weeks and signs of cholestasis median 2.5 weeks before peak SGPT values were reached. Extrahepatic manifestations occurred throughout the prodromal stage, the presence of arthropathy, urticaria or skin rashes was not related to the biochemical severity of liver disease. The higher the the maximal values of serum bilirubin and/or the older the patient, the longer the period of bilirubin elevation; a maximal bilirubin elevation less than 20 X the upper limit of normal was associated with normalisation of serum bilirubin within 6 weeks. No such correlations were found between the height of serum glutamic pyruvic transaminase, alkaline phosphatase, thymol turbidity and cholesterol levels and the subsequent duration of their abnormality. The elevation of alkaline phosphatase as well as the abdominal complaints might partly be caused by gastro-intestinal involvement. Immobilisation before peak SGPT was attained was associated with normalisation of serum glutamic pyruvic transaminase levels within 8 weeks after peak levels. 37 patients recovered completely. In one HBs-antigenemia and slight SGPT elevation persisted. Long term follow up was possible in 33 patients for 4 to 7 years, median 5 years.