[INTRODUCTION - The majority of the viral hepatitis is caused by five hepatitis viruses (A, B, C, D, E). In 1995, three new flaviviruses were discovered, the GBV-A, GBV-B and GBV-C (also known as HGV) viruses. The TT virus was discovered in 1997. Based on literature data, it is now supposed that a part of the unknown hepatitis cases is caused by the recently discovered hepatitis G or TT virus. The aim of this study was to determine the occurrence of hepatitis G and TT viruses in Hungary. PATIENTS AND METHODS - To reveal the RNA of the HGV viruses in the sera of patients suffering from hepatitis of unknown origin, RT-PCR was applied using the primers published in the literature. Seminested PCR was used to detect the DNA of TTV. The nucleotide sequences of nested PCR products were determined. Anti-HGV antibodies were detected by ELISA. RESULTS - The sera of 408 healthy persons older than 60 years were tested for the presence of hepatitis G virus antibodies: 113 tested positive. HGV virus antibodies were found in the sera of patients suffering from hepatitis of unknown origin or aplastic anaemia. 51 sera were tested and 20 were found to be positive for GBV-C antibodies, 4 for HGV RNA. Altogether, 213 sera of patients suffering from hepatitis of unknown origin or from aplastic anaemia were tested for HGV RNA and 26 were found to be positive. Eight PCR products were sequenced, and these sequences were found to be different from each other. 154 sera of patients with hepatitis of unknown origin were tested for the presence of TTV-DNA and 72 of them were positive. Seven PCR products were directly sequenced. Genotype 2 was found to be the most frequent one in Hungary.CONCLUSION - Our results show that both HGV and TTV are present in Hungary and none of them can be considered rare. Further studies are needed to reveal the association between the genotypes of these viruses and hepatitis of unknown origin.]