The course of choric hepatitis B is charaeterized by clinical and biochemical exacerbation and remission. Clearance rate of HBeAg can be different at different geographic areas, and has been reported to be higher in older patients. The clearance of HBeAg is also usually preceded by biochemical exacerbation and followed by biochemical remission. Therefore, high ALT level can be a predictor of imminent clearance of HBeAg. However, the clearance rate of HBeAg and its affecting factors have not been determined in Korea. Therefore, we conducted a prospective, longitudinal study to analyze the HBeAg clearance rate, and its relationship to biochemical remission in 102 HBeAg- positive Korean patients with chronic hepatitis B virus infection. They were followed over a period of 1-3 yr with regular checkup for serum HBeAg and biochemical liver function tests every 1-3 months. The probability of clearing HBeAg has a tendency to increase with the age of the patients; the cumulative probabilities of HBeAg clearance for the patients over 40 years of age were 24.7%, 44.2% and 62.1% at the end of 1, 2 and 3 years, respectively. The mean annual cumulative probability of HBeAg clearance was 16% for all 102 patients, and was significantly higher especially in patients whose initial ALT levels were above or equal to 120 IU/L (22%) than in those whose initial ALT levels below 120 IU/L (14%). Of 39 patients who cleared HBeAg, 25 patients (64%) experienced biochemical remissions at the time of HBeAg clearance, and furthermore, out of the rest 14 patients, 13 patients showed biochemical remission within 12 months following HBeAg clearance. These data suggest that the mean annual cumulative rate (16%) of spontaneous HBeAg clearance in Korea is in accordance with those of other Asian countries, and the high inital ALT level (above x 3 of normal) can be regarded as an independent clinical variable for predicting the high probability of HBeAg clearance as well as the age of the patient. We also confirmed a high biochemical remission rate in patients who cleared HBeAg.