The SEN virus (SEN-V) belongs to a recently discovered group of DNA viruses whose members (SEN-V-D and SEN-V-H) are associated with post-transfusion hepatitis. It is a single-stranded circular, non-enveloped DNA virus of approximately 3600 to approximately 3800 nucleotides with at least three open reading frames (ORFs). Eight different strains of SEN-V have been identified and provisionally classified as members of the Circoviridae family, a group of small, single-stranded, non-enveloped circular DNA viruses that includes the TT virus (TTV), TUS01, SANBAN, PMV and YONBAN. Prevalences in different populations show great variability with marked differences between different countries and groups. Although parenteral transmission is very likely, other routes of transmission cannot be excluded. Mother to infant transmission has been demonstrated. The effect of SEN-V on chronic liver diseases has been studied. The influence of SEN-V on the response to HCV therapy was investigated in three studies, with contradictory results. Data for other acute and chronic liver diseases are sparse. Further studies are needed to define the pathogenesis and clinical importance of SEN-V infection.