Hepatitis B virus (HBV) infection is a global health problem, with more than 350 million people chronically infected worldwide. The chronic HBV infection in Poland is also an essential medical and social problem. Starting from 1993, a steady decline of the incidence of HBV has been observed, reaching the estimated rate of 4.5 per 100 000 in 2004. Nothing is known about the genetic variability of HBV in Poland, the occurrence and spreading of genetic variants and mutants of hepatitis B virus in the population of Polish patients during the course of the disease and in relation to antiviral treatment. It is very interesting to study the molecular epidemiology of the Polish population regarding hepatitis B virus infection as Poland is still ethnically a uniform country, with no more than 3-4% of ethnic minorities. The first results regarding distribution of HBV genotypes and serotypes in northern Poland have been published by our group in 2003 and 2004. This work was part of a scientific project supported by the Fifth Framework Programme initiative of the European Union, entitled "Emerging variants of hepatitis B virus: new tools for epidemiological survey, diagnosis of infection, and monitoring of drug resistance". In the course of the project more than 200 hepatitis B infected patients from the northern part of Poland have been enrolled, diagnosed and - if the viral load of HBV was suitable for analysis - genotyped by sequencing of the HBV pol/S gene fragment. This review presents the main characteristics and some interesting aspects of the studied cohort of chronically infected patients from northern Poland as well as the molecular epidemiology.