In chronic hepatitis B, it is difficult to predict an early therapeutic response. We investigated the viral decline during therapy with pegylated interferon alpha-2b (PEG-IFN) with or without lamivudine in 266 HBeAg-positive chronic hepatitis B patients. In patients treated with PEG-IFN and lamivudine, a uniform biphasic viral decline pattern was found during therapy and there were no marked differences in viral load between those who lost HBeAg at the end of follow-up (response) or not. In contrast, those treated with PEG-IFN monotherapy exhibited different viral decline patterns. A delayed decline of at least two log from baseline HBV DNA after week 4 but before week 32 was associated with the highest response rate (63%). In comparison, response was 52% for patients with an early decline (week 0-4), 38% for a late decline (week 32-52), 27% for a posttreatment decline (week 52-78) and 11% for patients with no substantial decline. The HBsAg loss was 22% in the delayed decline pattern compared to 4% for those with early decline and none for other decline patterns. In conclusion, different patterns of decline in viral load during treatment with PEG-IFN monotherapy were associated with different rates of HBeAg and HBsAg loss at the end of follow-up. Since there was a considerable response, even in patients with a late or posttreatment decline pattern, prediction of response based on viral decline during the first months of therapy was difficult.